v0.5.0

Note: this is supposed to be essentially a 1.0 release candidate. If there are no problems with this release, it will be released as 1.0 after some time.

• Breaking Bump MSRC from 1.54 to 1.56
• Breaking: Remove proc-macro2 from default features
• Breaking: Make StringLit<String>::into_value return String instead of Cow<'static, str>.
  Make ByteStringLit<String>::into_value return Vec<u8> instead of Cow<'static, [u8]>.
• Add support for C string literals
• Limit number of # as delimiter of raw string literals to 256 (to align with spec)
• Fix license in Cargo.toml to use proper SPDX format (in #19, thanks @atouchet)
• Fix error span for InvalidStartOfUnicodeEscape errors