v0.7.0

Changed

• Add Lexer::from_str as the safe instantiation method.
• Refactor Lexer::from_slice to unsafe Lexer::from_slice_unchecked. It is
  assumed the byte slice is UTF-8 characters, and if they are not a valid slice
  of UTF-8 bytes, then the behavior is undefined.
• Dynamically determine Token type instead of storing the Ty. This reduces the
  Token size to 2 words (16 octets on a 64-bit machine).

Removed

• Remove offset field from Token. If required, the offset should be
  calculated when tokens are consumed.
• Remove scan and Scanner from the public API.
• Remove const from Token::ty() method for possible future optimizations.
• Remove public new methods from Token and related types.

Added

• Add unsafe as_str_unchecked to various types in cases where the bytes are
  guaranteed to be valid UTF-8.
• Add as_bytes, to_str, as_str_unchecked, and into_inner to Token.