Refactor checker to use Nature (#665)

This PR introduces a new concept inside Expr's type checker - Nature.

It's an abstraction other reflect.Type which carries additional information about possible type, methods and builtin function.

I was able to find and fix a few bugs in type checker and optimizer. Implementation of nature concept has proven itself. As well nature will allow to bring more feature for type checker in future.

Why Nature? I was looking for a name to replace Type. Type name already carries much information, as well can be confused with reflect.Type. Nature name should be understood as "nature of the node", same as "type of the node".

testdata/examples.txt

@@ -2041,7 +2041,6 @@
 0.5 < 1 || ok
 0.5 < f32 && ok
 0.5 < f32 || ok
-0.5 <= 0.5 ? half : trimPrefix(nil)
 0.5 <= f64 ? i : i64
 0.5 <= f64 || ok
 0.5 == 1 and ok
@@ -3144,7 +3143,6 @@ all(reduce(array, array), # > #)
 all(reduce(array, list), ok)
 all(true ? list : false, not true)
 any(["bar"], # not endsWith #)
-any([foo], # != i64)
 any([greet], ok)
 any(array, !(i32 <= #))
 any(array, "foo" >= "foo")
@@ -6261,7 +6259,6 @@ findLast(map(list, #), greet == nil)
 findLast(map(list, #), ok)
 findLast(map(list, ok), # ? # : #)
 findLastIndex(1 .. 1, i64 < #)
-findLastIndex([false], 0.5 == #)
 findLastIndex([i64, half], ok)
 findLastIndex(array, "bar" not matches "foo")
 findLastIndex(array, "bar" startsWith "foo")
@@ -7231,7 +7228,6 @@ get(false ? f64 : 1, ok)
 get(false ? f64 : score, add)
 get(false ? false : f32, i)
 get(false ? i32 : list, i64)
-get(false ? i64 : foo, Bar)
 get(false ? i64 : true, f64)
 get(false ? score : ok, trimSuffix("bar", "bar"))
 get(filter(list, true), i)
@@ -7370,27 +7366,19 @@ get(map(list, i32), i64)
 get(map(list, i64), i64)
 get(ok ? "bar" : false, array)
 get(ok ? 1 : "bar", f32)
-get(ok ? 1 : add, String)
 get(ok ? add : f32, array)
-get(ok ? array : i, Bar)
-get(ok ? f32 : list, get(half, nil))
 get(ok ? f64 : div, i64)
-get(ok ? f64 : list, half in Bar)?.array()
 get(ok ? false : list, f32 > i)
 get(ok ? foo : 0.5, div)
-get(ok ? greet : "bar", get(add, String))
 get(ok ? half : i32, i)
 get(ok ? half : ok, f64)
 get(ok ? i : 0.5, half)
-get(ok ? i : half, String)
 get(ok ? i32 : half, f64)
 get(ok ? i64 : foo, f32)
 get(ok ? list : "foo", add)
 get(ok ? list : i32, f32)
 get(ok ? ok : div, greet)
-get(ok ? score : 1, Qux?.i)
 get(ok ? score : f64, i)
-get(ok ? score : foo, String?.foo())
 get(ok ? score : i64, foo)
 get(reduce(list, array), i32)
 get(sort(array), i32)
@@ -7398,19 +7386,14 @@ get(take(list, i), i64)
 get(true ? "bar" : ok, score(i))
 get(true ? "foo" : half, list)
 get(true ? 0.5 : i32, array)
-get(true ? 1 : array, Bar)?.f64
 get(true ? f32 : 0.5, ok)
 get(true ? false : foo, i64 > 0.5)
-get(true ? greet : false, Bar)
 get(true ? greet : i32, score)
-get(true ? half : 0.5, Qux)
 get(true ? half : f32, greet)
 get(true ? half : list, add)
 get(true ? i64 : greet, i32)
 get(true ? score : true, half)?.half
-get(true ? true : add, Qux)
 get(true ? true : i, f64)
-get({"bar": true, "foo": greet}.i, String)
 get({"foo": foo, "bar": false}, type(i))
 greet
 greet != add
@@ -12465,7 +12448,6 @@ map(1 .. i64, # ^ #)
 map(1 .. i64, #)
 map(1 .. i64, half)
 map(1 .. i64, i32)
-map([1], get(#, 1))
 map([f64], half)
 map([false], ok)
 map([half], #)
@@ -12475,7 +12457,6 @@ map([i32], foo)
 map([i32], greet)
 map([i32], half)
 map([list, 1, foo], i32)
-map([nil], #?.f32(ok, #, false))
 map([nil], foo)
 map([score, "bar"], f32)
 map([true, i32, 1], #)
@@ -14870,7 +14851,6 @@ ok ? array : foo
 ok ? array : foo.String
 ok ? array : foo?.String
 ok ? array : greet
-ok ? array : greet(nil)
 ok ? array : i
 ok ? array : i32
 ok ? array : list
@@ -14890,7 +14870,6 @@ ok ? div : ok
 ok ? div : score
 ok ? f32 != i32 : f32
 ok ? f32 ** i64 : score
-ok ? f32 : "foo" endsWith lower(nil)
 ok ? f32 : -0.5
 ok ? f32 : add
 ok ? f32 : array
@@ -15104,7 +15083,6 @@ ok or f64 <= 1
 ok or false ? ok : 1
 ok or false and false
 ok or foo != foo
-ok or fromJSON(nil)?.ok
 ok or half != add
 ok or half == div
 ok or i < 1
@@ -15340,29 +15318,15 @@ reduce(1 .. i64, greet)
 reduce(1 .. i64, half)
 reduce(["foo"], f32)
 reduce([0.5], f64)
-reduce([1], .Qux)
 reduce([div, "foo"], i32)
-reduce([div], # >= 0.5)
-reduce([div], list == #)
 reduce([f32, f64], f32)
-reduce([f32], .f64)
-reduce([f64], # or #)
-reduce([f64], .greet)
-reduce([false], # ** #)
 reduce([foo], ok)
-reduce([greet], # % 1)
 reduce([i32], list)
 reduce([i64, half], "bar" not startsWith "foo")
 reduce([i], # < #)
-reduce([i], #[array])
 reduce([i], ok)
-reduce([list], # - #)
-reduce([list], # or #)
 reduce([list], f64)
 reduce([nil], # in array)
-reduce([ok], #?.half)
-reduce([true], #?.score())
-reduce([true], 0.5 > #)
 reduce(array, !false)
 reduce(array, "foo" endsWith "foo")
 reduce(array, "foo") not in foo