Support for no-alloc environments

[TechnoPorg]

bytesize currently assumes that the alloc crate will be available, with no feature flags to toggle it on or off:

bytesize/src/lib.rs

Lines 46 to 48 in 0121741

	extern crate alloc;
	use alloc::string::ToString as _;

However, I'd like to use this in an embedded environment, where alloc is genuinely not available, and I don't need the formatting capabilities it would offer. It would be nice if you could gate that behind an "alloc" feature flag, which would then be pulled in by the std feature flag. Thanks!

[robjtede]

What do you think about #141 ?

Without a semver-major change, it's possible to retain the Display impl, but not the parsing since the Error type is String.