Add missing label support for entity_statement.

This is additional commit for pull request #30.
Fix scope of entity_statement_part.

This is an additional commit for pull request #30.
Test semantics of conc_assertion_stmt in entity_stmt_part.

This is an additional commit for pull request #30.
Update test/regress/assert3.vhd.

* Add label.
* Add stub architecture to run.

This is an additional commit for pull request #30.
Test assert3 regression w/ gold.

This is an additional commit for pull request #30.