microstr 0.3.0

# ChangeLog

## 0.3
### Added:
- fmt support
- methods: `from_const`, `truncate`
- const support for macro
### Changed:
- Signatures of `from_str`, `push_str`
- Very small methods become `#[inline]`
### Removed:
- private method `push_bytes`

## 0.2
### Added:
- Macro `microstr!`
- Method `.clear()`
### Changed:
- Docs to `microstr!` using

## 0.1
- Initial version