Implement functionality present on String and SmallVec

[daboross]

This adds a large amount of functionality to SmallString.

I've found the library very useful, so I thought this would be a good way to contribute back.

Most methods which make sense from String and SmallVec are now implemented on SmallString.

This PR:

• removes default size = [u8; 8] to match behavior of SmallVec
• implements many methods available on SmallVec but not SmallString
• implements AsMut without feature gate and without requiring rust 1.20+
• implements Extend efficiently with pre-reserved capacity
• re-implements FromIterator more simply
  • the previous FromIterator implementation worked well and was efficient, but it's possible to achieve the same result without creating a whole struct implementing Iterator. The new behavior matches String::from_iterator.
• updates smallvec crate dependency to achieve full functionality
• implements many more traits which can be implemented
• implements some From implementations more efficiently using direct byte-for-byte copies rather than collect() on an iterator.
• bumps version to '0.2.0' to prepare for release.

Fixes #1, supersedes #2, #3.