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/jsgen.nim

@@ -167,6 +167,8 @@ proc mapType(typ: PType): TJSTypeKind =
      tyNone, tyFromExpr, tyForward, tyEmpty, tyFieldAccessor,
      tyExpr, tyStmt, tyTypeDesc, tyTypeClasses, tyVoid, tyAlias:
     result = etyNone
+  of tyInferred:
+    result = mapType(typ.lastSon)
   of tyStatic:
     if t.n != nil: result = mapType(lastSon t)
     else: result = etyNone