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/types.nim

@@ -1195,7 +1195,8 @@ proc sameTypeAux(x, y: PType, c: var TSameTypeClosure): bool =
     cycleCheck()
     result = sameTypeAux(a.lastSon, b.lastSon, c)
   of tyNone: result = false
-  of tyOptDeprecated: doAssert false
+  of tyConcept:
+    result = exprStructuralEquivalent(a.n, b.n)
 
 proc sameBackendType*(x, y: PType): bool =
   var c = initSameTypeClosure()
@@ -1271,6 +1272,7 @@ proc matchType*(a: PType, pattern: openArray[tuple[k:TTypeKind, i:int]],
     a = a[i]
   result = a.kind == last
 
+
 include sizealignoffsetimpl
 
 proc computeSize*(conf: ConfigRef; typ: PType): BiggestInt =