Adds new nodes to AST for type decls

As yet the code generator is not using these nodes.
Work in progress.

Perhaps mistakenly, replaced series of if with elseifs in
identifier resolution.
Work in progress.

This version generates code using the new AST nodes and new code
generation methods in genc.grace.  The code runs, at least for a simple
example.

Some files contain debugging printouts.  This version has not been
tested against any test suite.  However, it did find some errors in
StandardPrelude (repeated type declarations) that the old version
missed, so it's not all bad.

No work yet done on genjs.grace, but that should be much simpler
than genc.grace

Includes simple test program tests/t15a_typeCompile_test.grace.
Improves error message for parameter name clash.

Offending name now comes first.