Absolute path support in the emulator

[greenhat]

Extracted from #222 (comment)

We may need to extend our representation a bit, in order to allow importing MASM modules from source. In particular, we may want to have a variant of FunctionIdent called Callee used in our MASM representation, like so:

pub enum Callee {
    MastRoot(RpoDigest),
    Path {
        module: Option<Ident>,
        function: Ident,
        absolute: bool,
    }
}

When translating HIR FunctionIdent to Callee, it would become a path where the module would always be set, and absolute would always be true (since function identifiers in HIR are always absolute).

When translating MASM InvocationTarget to Callee, we'd convert InvocationTarget::AbsolutePath as an absolute callee, InvocationTarget::ProcedurePath as a relative path, InvocationTarget::ProcedureName as a relative path, and InvocationTarget::MastRoot, as a MAST root. Similar translations could be supported for other InvocationTarget-like procedure identifiers, but the MASM opcodes only use InvocationTarget.

The emulator then would need to resolve relative paths at runtime, but could treat absolute paths and MAST roots as, well, absolute references. The emulator currently does not track MAST roots at all, so that would be needed to have full compatibility, but would be a pretty simple thing to add.