no_std

[fralalonde]

I'm using utf16string for USB descriptor parsing in an embedded context, where use of std is usually not possible.

This change introduces two build features:

• std (enabled by default)
• alloc

Using the crate in plain no_std is thus now possible with default-features = false. The alloc feature enables some features like WString in when the no_std environment defines an allocator

Since std is marked as a default feature, this change should be completely transparent to existing users of the crate. All tests confirmed passing.

Summary of code changes:

• references to std::* replaced by their core::* version where possible
• references to std::* with no core::* equivalent gated by std feature (mainly std::error::Error)
• code requiring allocation gated by std or alloc features