Fill in dropped annotations and unaccounted attributes in the frontend

[simonspies]

Closes the frontend gaps where an annotation or an attribute was silently dropped, and makes local recursion usable.

• A type annotation written on a let pattern (let (x : T) = e) or on a match-arm payload is honored instead of discarded, so a wrong type is caught and a [@spec] written there is not lost.
• An annotation on the name of a let that takes arguments is rejected rather than dropped, since the annotation there is the return type written after the arguments (and OCaml rejects the syntax outright).
• A let with no arguments that is annotated both on its binder and after it is rejected, instead of the second annotation being silently dropped.
• Every attribute on a value declaration is accounted for: an unknown name is rejected, and neither [@@spec] nor [@@fn] may be written twice.
• An attribute carries its own source location, so an attribute error points at the attribute rather than at the declaration or at :0:0.
• A local let rec binding a function literal is supported, which was the only way to give a local recursive function a specification and so the only way to write one at all.
• A declaration's attributes are collected in a record rather than threaded as separate arguments and returned as a tuple.
• Attribute names are a variant type rather than matched as strings.
• Two gaps are pinned by tests rather than fixed: a type assignment (e : t) is still dropped (Support type assignments (e : t) #155) and () is still rejected in a binder position (Accept () in a binder position #156).