top-down type inference, implements rfc 149 (#20091)

* micro implementation of rfc 149

refs https://github.com/nim-lang/RFCs/issues/149

* number/array/seq literals, more statements

* try fix number literal alias issue

* renew expectedType with if/case/try branch types

* fix (nerf) index type handling and float typed int

* use typeAllowed

* tweaks + const test (tested locally) [skip ci]

* fill out more of the checklist

* more literals, change @ order, type conversions

Not copying the full call tree before the typedesc call check
in `semIndirectOp` is also a small performance improvement.

* disable self-conversion warning

* revert type conversions (maybe separate op later)

* deal with CI for now (seems unrelated), try enums

* workaround CI different way

* proper fix

* again

* see sizes

* lol

* overload selection, simplify int literal -> float

* range, new @ solution, try use fitNode for nil

* use new magic

* try fix ranges, new magic, deal with #20193

* add documentation, support templates

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

tests/stdlib/tsequtils.nim

@@ -453,8 +453,8 @@ block:
       for i in 0..<len:
         yield i
 
+  # xxx: obscure CT error: basic_types.nim(16, 16) Error: internal error: symbol has no generated name: true
   when not defined(js):
-    # xxx: obscure CT error: basic_types.nim(16, 16) Error: internal error: symbol has no generated name: true
     doAssert: iter(3).mapIt(2*it).foldl(a + b) == 6
 
 block: # strictFuncs tests with ref object