introduce tyInferred for the unbound concept type params

* Why is tyInferred needed?

The bindings in TCandidate are capable of inferring types within a single
call expression. In concepts, we need to infer types in the same way, but
across the whole body of the concept.

Previously, once a concept type param was inferred, it was destructively
mutated using t.assignType, but this proved to be problematic in the presence
of overloads, because the bindings established while a non-matching overload
is tested must be reverted/forgotten. tyInferred offers a non-destructive way to
keep track of the inference progress.

While introducing new types usually requires a lot of code paths in the compiler
to updated, currently tyInferred is only a short-lived type within the concept body
pass and it's unlikely to introduce breakage elsewhere in the compiler.

compiler/ccgtypes.nim

@@ -165,7 +165,7 @@ proc mapType(typ: PType): TCTypeKind =
   of tyOpenArray, tyArray, tyVarargs: result = ctArray
   of tyObject, tyTuple: result = ctStruct
   of tyGenericBody, tyGenericInst, tyGenericParam, tyDistinct, tyOrdinal,
-     tyTypeDesc, tyAlias:
+     tyTypeDesc, tyAlias, tyInferred:
     result = mapType(lastSon(typ))
   of tyEnum:
     if firstOrd(typ) < 0: