Project format needs to change to accomodate user-provided signatures

[uxmal]

Realisitically, most users will be providing signature definitions in a C-like syntax:

int foo([reko:reg("eax")] arg1)

The current project file format doesn't support this. It assumes all signature definitions are fine-grained expressions in XML:

<signature><ret><reg>eax</reg><ret><arg name="arg1"><reg>eax</reg></arg></signature>

No rational human will put up with that in the long run.

To do: extend the file format to support both kinds of definitions.

[uxmal]

implemented