Remove unused variables.
Remove an incorrect comment.

The type of a ConstSpec isn't inferred, it is simply untyped.
Move check.go to decl.go

This file is all about declarations.
Return an error on multiple declarations, and don't bind _ to a declaration.

Also, a couple other cleanups:
1) move errors into errors.go
2) rename check.go to decl.go—it's all about declarations.
Gather import bindings.

The imports aren't actually loaded, but their names are bound in the file-level symbol tables.
Merge pull request #36 from velour/front

Some cleanups, report redeclaration errors, and build file-level symbol tables.