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/ccgutils.nim

@@ -110,7 +110,7 @@ proc getUniqueType*(key: PType): PType =
     of tyDistinct:
       if key.deepCopy != nil: result = key
       else: result = getUniqueType(lastSon(key))
-    of tyGenericInst, tyOrdinal, tyStatic, tyAlias:
+    of tyGenericInst, tyOrdinal, tyStatic, tyAlias, tyInferred:
       result = getUniqueType(lastSon(key))
       #let obj = lastSon(key)
       #if obj.sym != nil and obj.sym.name.s == "TOption":