Add no_std support

[TTWNO]

Hi there,

I'm working on a project to get an extremely minimal TTS working in Rust.
As part of the requirements, my library needs to be #![no_std].
It'd be nice to take advantage of existing libraries; and yours is very easily adapted to no_std.
This makes that simple change.

The only two future consideration are:

• anything that does depend on std will need to be behind a feature flag, and
• you may want to enable std_instead_of_alloc; this will suggest that any imports from std:: be changed to alloc:: where possible.

Thanks for putting in the hard work for this!

[Ballasi]

Hey, thanks for opening that PR!

I am relatively busy lately so I might take a bit of time to take a look and answer your PR (the other PR dates back from 1+ month which is completely horrendous...), even though it doesn't look that complicated.

Thanks for taking the time to participate in this project. I'll answer ASAP.

[Laifsyn]

I'm not in particular hurry since I was planning to see if I could change from a loop into an iterator
Update: I couldn't make it work with an Iterator. (I wanted to reduce allocations mainly)

[TTWNO]

@Ballasi just checking in. No rush, I just had to use my git version of the project again for a no_std target and wanted to see if progress could be made regarding a review.