Exposed safe blst_scalar_constructor

[ayushmishra2005]

Fixes #13

[dot-asm]

bindings.rs is auto-generated by bindgen and it's absolutely preferred to keep it that way. It's not unmodified output though, it's passed through build/bindings_trim.pl. Besides, scalars are now byte arrays, so that subroutines in question are of lesser use. In sense that if you have a collection of random bytes you can just pass them without having to bother about conversions. Accompanied with suitable nbits of course.

On side note, one can make a case for nbits sanity check for relation between nbits and slice length, or even nbits inference [from slice length].