Add more members of filter_parser to milli:: & From<&str> implementation for Token

[GregoryConrad]

What does this PR do?

The current milli::Filter and milli::FilterCondition APIs require working with some members of filter_parser directly that milli:: does not re-export to its users (at least when not parsing input using parse). Also, using filter_parser does not make sense when using milli from an embedded context where there is no query to parse.

Instead of reworking milli::Filter and milli::FilterCondition, this PR adds two non-breaking changes that ease the use of milli:

• Re-exports more members of the dependent version of filter_parser in milli
• Implements From<&str> for filter_parser::Token
  • This will also allow some basic tests that need to create a Token from a string to avoid some boilerplate.

In conjunction, both of these will allow milli users to easily create a Token from a &str without needing to add filter_parser as an extra dependency.

Note: I wanted to use FromStr for the From implementation; however, it requires returning a Result which is not needed for the conversion. Thus, I just left it as From<&str>.

[irevoire]

That's good for me.
I'll let @Kerollmops merge if he's ok with the PR as well.

[Kerollmops]

Thank you very much. Looks good to me, merging!
bors merge

[bors]

Build succeeded:

• Run Clippy
• Run Rustfmt
• Tests on macos-latest
• Tests on ubuntu-20.04
• Tests on windows-latest