Fix bug referencing const declarations in other modules.

[otrho]

Caveat, line 1239 in mod.rs:

    // NOTE: this is currently flattening out the entire namespace, which is problematic.  To fix
    // it we need to support hierarchical names (or at least absolute normalised names) to
    // AsmNamespace.  This can be done in the new ASM generator which translates from IR, coming
    // soon.

Closes #297.

[sezna]

LGTM