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>

compiler/varpartitions.nim

@@ -494,7 +494,9 @@ proc destMightOwn(c: var Partitions; dest: var VarIndex; n: PNode) =
         # this list is subtle, we try to answer the question if after 'dest = f(src)'
         # there is a connection betwen 'src' and 'dest' so that mutations to 'src'
         # also reflect 'dest':
-        if magic in {mNone, mMove, mSlice, mAppendStrCh, mAppendStrStr, mAppendSeqElem, mArrToSeq}:
+        if magic in {mNone, mMove, mSlice,
+            mAppendStrCh, mAppendStrStr, mAppendSeqElem,
+            mArrToSeq, mOpenArrayToSeq}:
           for i in 1..<n.len:
             # we always have to assume a 'select(...)' like mechanism.
             # But at least we do filter out simple POD types from the