Define traits to regulate ItemFn, Signature, Ident

[ariasanovsky]

Specifically:

• prohibit Ident's that start with __ (one _ is okay) and ones ending with _
• if an ItemFn's Ident starts with _, use the trimmed name for our internally defined fn's & structs
• we aren't being systematic in restricting const-ness, pub, safe/unsafe, etc

[ariasanovsky]

Incorporated progress into the impl Parse for MapFn block.