[feature] Inter-format-string splicing

[ToxicFrog]

It is often convenient to incrementally build a format string, creating format strings describing individual sub-records before combining them into something larger. This is currently unwieldy and must be done with .. or string.format.

It would be nice to be able to do something like, say:

vstruct.register("toc_entry", "path:c4 size:u4")
vstruct.register("toc", "count:u4 toc:{ $count * <toc_entry> }")
vstruct.unpack("<toc>", buffer)

Syntax is still a work in progress, and this can be considered an intermediate step towards format string combinators (which I need to open an issue for).

[ToxicFrog]

848fd72 implements this, although the syntax and API are still a work in progress; I'm not ready to mark this closed (or cut a new release) until I'm happy with them.

Syntax uses &name, since $name is reserved for future expansion (backreferences).