Add command for loading module

[xiaq]

The use special command does two things: it 1) loads a module, and 2) associate it with a name. Because it does 2, and due to lexical scoping, it cannot take variable module names.

We need a command that only does 1) and is able to take variable module names. Let's say that the command is load-mod; then the following two lines should be equivalent:

use mod
mod: = (load-mod mod)

However, the difference is that load-mod can take a variable argument:

mod: = (load-mod $var)

[xiaq]

This has been implemented as use-mod.