Support for explicit regions in types

[melsman]

This PR deals with issue #124 and implements a leftover from #34, namely to provide support for region annotations in types:

ty ::= ...
       | t`r | {lab:ty,...,lab:ty}`r
       | (ty * ty)`r | (ty -> ty)`r

Here is a more detailed implementation task list:

• Extend Topdec.grm to support the extended grammar and extend DecGrammar.sml accordingly.
• Allow for explicit regions to appear in semantic types (unification should propagate explicit region information) by changing code in StatObject.sml.
• Extend ElabDec.sml to insert explicit regions into constructed semantic types.
• Extend CompileDec to compile region annotations in types into region annotations in LambdaExp types.
• Make sure that OptLambda preserves the type annotations when performing optimisations, which can be tricky.
• Many tests in test/explicit_regions/.