ArrayString lacks removal features

[WildCryptoFox]

ArrayString lacks any removal features. pop truncate remove etc would be nice.

All of these may be copied from std::String AFAIK.

I will implement these and more if requested. I've opened this issue to discuss because this seems too obvious to have been accidentally omitted?

[tbu-]

These are not omitted on purpose.

[WildCryptoFox]

@tbu- I would expect not. I'll implement this "soon (tm)" unless someone beats me to it.

[bluss]

Mentoring instructions: PRs are welcome.

• ArrayString is completing its API towards being more like String.
• Focus on mimicking existing String methods in their complete API (equal signature is preferred) and in docs (adapted appropriately).
• Ask me on rust channels on mozilla IRC if you want