new-style concepts implementation, WIP (#15251)

* fixes #15210 [backport:1.2]

* make tests green
* make ordinal work
* makes Swapable test compile
* make Indexable example work
* concepts: 'self' is now 'Self'
* concepts: make Dictionary example compile
* document the new concept implementation
* concepts: make typeDesc work properly
* concepts: allow documentation comments (d'oh)

compiler/typeallowed.nim

@@ -187,7 +187,9 @@ proc typeAllowedAux(marker: var IntSet, typ: PType, kind: TSymKind,
       result = typeAllowedAux(marker, t.lastSon, kind, c, flags+{taHeap})
     else:
       result = t
-  of tyOptDeprecated: doAssert false
+  of tyConcept:
+    if kind != skParam: result = t
+    else: result = nil
 
 proc typeAllowed*(t: PType, kind: TSymKind; c: PContext; flags: TTypeAllowedFlags = {}): PType =
   # returns 'nil' on success and otherwise the part of the type that is