Add TinyAsciiStr::concat

[sffc]

5729

[sffc]

I considered a few other signatures. I picked this one because:

1. If the input were a &str, we'd need to check each character again. If you want to append a &str, you can first convert it to a TinyAsciiStr of the same size as the output.
2. concat is more general than some of the alternatives. For example, to push a single character, you can make a TinyAsciiStr<1> and then call concat.
3. To modify the receiver, you can save the result back, as in *x = x.concat(y). Note that &mut self is not const.
4. I saw no good reason to enforce that Q == M + N because resize already is documented to do the truncation thing, and it saves the client from having to call resize themselves.

[sffc]

@zbraniecki You can submit this (to unblock your PR) if you agree with my logic about not adding the assertion.

[zbraniecki]

I'm fine merging it as is. No blockers.