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/ast.nim

@@ -674,7 +674,7 @@ type
     mInSet, mRepr, mExit,
     mSetLengthStr, mSetLengthSeq,
     mIsPartOf, mAstToStr, mParallel,
-    mSwap, mIsNil, mArrToSeq,
+    mSwap, mIsNil, mArrToSeq, mOpenArrayToSeq,
     mNewString, mNewStringOfCap, mParseBiggestFloat,
     mMove, mWasMoved, mDestroy, mTrace,
     mDefault, mUnown, mFinished, mIsolate, mAccessEnv, mAccessTypeField, mReset,
@@ -708,8 +708,8 @@ type
     mSymIsInstantiationOf, mNodeId, mPrivateAccess
 
 
-# things that we can evaluate safely at compile time, even if not asked for it:
 const
+  # things that we can evaluate safely at compile time, even if not asked for it:
   ctfeWhitelist* = {mNone, mSucc,
     mPred, mInc, mDec, mOrd, mLengthOpenArray,
     mLengthStr, mLengthArray, mLengthSeq,
@@ -738,6 +738,9 @@ const
     mEqSet, mLeSet, mLtSet, mMulSet, mPlusSet, mMinusSet,
     mConStrStr, mAppendStrCh, mAppendStrStr, mAppendSeqElem,
     mInSet, mRepr}
+  
+  generatedMagics* = {mNone, mIsolate, mFinished, mOpenArrayToSeq}
+    ## magics that are generated as normal procs in the backend
 
 type
   ItemId* = object
@@ -1677,6 +1680,10 @@ proc transitionIntKind*(n: PNode, kind: range[nkCharLit..nkUInt64Lit]) =
   transitionNodeKindCommon(kind)
   n.intVal = obj.intVal
 
+proc transitionIntToFloatKind*(n: PNode, kind: range[nkFloatLit..nkFloat128Lit]) =
+  transitionNodeKindCommon(kind)
+  n.floatVal = BiggestFloat(obj.intVal)
+
 proc transitionNoneToSym*(n: PNode) =
   transitionNodeKindCommon(nkSym)