Add #![no_std] compatibility

[ben-krieger]

• Add std feature flag to allow disabling std lib for no_std environments
• Add tests to ensure no_std support isn't broken by future changes
• Add documentation on how to use library in a no_std environment

Note: extern crate alloc is still required, but I didn't want to add another alloc feature flag, since virtually nothing would be possible without it.

[ben-krieger]

Note that this is the only function actually enabled with std. All other std usage is already behind svg and image flags.