provide awareness of alias values in debugger

There are some text-based alias terms such as "dos_filedescriptor_offset_currentcluster" that aren't in the symbols described in the "*.map" file.

They appear in the source in this form:

```
.alias dos_filedescriptor_offset_currentcluster 6
```

It'd be good to be able to parse such info out of the .a65 files and make use of it within the debugger and the readline library too.
