Add support for including slices of binary files

[MineRobber9000]

Adds 2 new forms of incbin:

• incbin(relative_filename, start): includes relative_filename starting from offset start
• incbin(relative_filename, start, size): includes size bytes from relative_filename starting from offset start

I'd also like to make incbinstr and inchexstr work similarly, but I'm not sure how to make it work (extra clarification for PR message that wasn't in the commit message: incbinstr and inchexstr go straight from a bitvec to a bigint with no intervening Vec<u8> step, so I can't slice it as easily as I can incbin).

When #191 gets merged, ensure_inc_args can be replaced with a call to query.ensure_min_max_arg_number(1,3)?;. Since I don't have that merged locally, I just wrote a quick and dirty function to do it.

[MineRobber9000]

In the process of writing the implementations for incbinstr and inchexstr, I've come to the realization that I'm not exactly sure what start and size ought to entail when used. For incbin, the obvious answer was "start and size are counted in bytes", but with incbinstr and inchexstr being able to include values that aren't a round number of bytes, what should start and size be measured in? Bits?

[hlorenzi]

Hmmm, I think going by the logic, incbinstr and inchexstr should be measured in characters (which, in the set we allow, are also just bytes). But there's the issue of having ignorable characters inside these files (like whitespace and underscores).

So they should probably be measured by the multiple of bits each character is converted into, 1 and 4 respectively. I think it can be done by directly slicing the final output bitvec.to_bigint().

[MineRobber9000]

I'd say I'm proud of this PR. Ready for merge!