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>
(cherry picked from commit 0014b9c48e)

compiler/sizealignoffsetimpl.nim

@@ -454,9 +454,9 @@ proc computeSizeAlign(conf: ConfigRef; typ: PType) =
     setSize typ, 1
   of tyInt16, tyUInt16:
     setSize typ, 2
-  of tyInt32, tyUInt32:
+  of tyInt32, tyUInt32, tyFloat32:
     setSize typ, 4
-  of tyInt64, tyUInt64:
+  of tyInt64, tyUInt64, tyFloat64, tyFloat:
     setSize typ, 8
   else:
     typ.size = szUnknownSize