Add SRFI 178 - Bitvector library

[jpellegrini]

Hi @egallesio !

Well... I thought I'd implement one more SRFI... :grin:

This is SRFI 178 - Bitvector library.

• Written mostly in C, but with some parts in Scheme

MISSING:

• The reader syntax, '#*', is not implemented, because the reader has no way to know if the SRFI has been loaded.

Regarding this, I see some possibilities:

• Adopt the bitvector type as native, and keep only bitvector, bitvector-set!, bitvector-ref in the core, and if the user wants, the rest may be brought with (import (srfi 178)).
• Use a global variable loaded_srfi_178 that tells the reader wether it can read bitvectors. However -- the bitvector data structure would not be known to the reader, and the SRFI would at least have to change a function pointer in the reader.
• Don't implement the read syntax (but it's very convenient; #*10011 instead of (bitvector 1 0 0 1 1))...

POSSIBLE OPTIMIZATIONS FOR THE FUTURE:

• Some list reversals may be unnecessary
• Some procedures go bit-by-bit, when they could iterate on bytes

[jpellegrini]

I forgot to mention: the read syntax is not mandatory according to the SRFI...

[jpellegrini]

Also... The code to read from a string is already there (see string->bitvector).

[jpellegrini]

Hi @egallesio ! I see you have implemented the #* syntax - that's great! :smile:

[egallesio]

Yep, code of the reader has been refactored for #xxx objects and can be extended in Scheme (see last comment in Issue #434). I hope it will be useful for future extensions.

Anyway, thanks a lot @jpellegrini for this great PR.