spec for view types (#15424)

* spec for view types
* spec additions
* refactoring; there are two different kinds of views
* refactorings and spec additions
* enforce that view types are initialized
* enforce borrowing from the first formal parameter
* enforce lifetimes for borrowing of locals
* typo in the manual
* clarify in the implementation what a borrow operation really is

compiler/ast.nim

@@ -1403,9 +1403,6 @@ proc copyType*(t: PType, owner: PSym, keepId: bool): PType =
 
 proc exactReplica*(t: PType): PType = copyType(t, t.owner, true)
 
-template requiresInit*(t: PType): bool =
-  t.flags * {tfRequiresInit, tfNotNil} != {}
-
 proc copySym*(s: PSym): PSym =
   result = newSym(s.kind, s.name, s.owner, s.info, s.options)
   #result.ast = nil            # BUGFIX; was: s.ast which made problems