In anticipation of future attributes, and perhaps the ability to
declare lvalues with specifiers other than 'let' and 'var', expand
the "isLet" bit into a more general "specifier" field.
With the introduction of special decl names, `Identifier getName()` on
`ValueDecl` will be removed and pushed down to nominal declarations
whose name is guaranteed not to be special. Prepare for this by calling
to `DeclBaseName getBaseName()` instead where appropriate.
* Allow CodingKey conformance to be automatically derived for enums
which have no raw type (with no associated values) and which have
a raw type of String or Int.
* Allow Encodable and Decodable conformance to be automatically derived
for classes and structs with Encodable/Decodable properties
* Add initial unit tests for verifying derived conformance
