Both ReturnStmt and DeclStmt now check the values being associated
with references to make sure that they are legal (e.g. it's illegal
to assign a varying lvalue, or a compile-time constant to a reference
type). Previously we didn't catch this and would end up hitting
assertions in LLVM when code did this stuff.
Mostly fixes issue #225 (except for adding a FAQ about what this
error message means.)
