Add alloc feature

[tormol]

Enables AsciiString in #![no_std]-mode by using the alloc crate instead of std.
Methods using CStr or CString, and impls of Error still require std.

Is it worth adding a feature for this? (or should AsciiString be always available or still require std feature?)

[tomprogrammer]

I think introducing an alloc feature in addition to the std feature can be useful on embedded platforms. Good work!