Change MonadState instance

[jamesdbrock]

state refers to base monad instead of ParseState

Resolves #56

With this PR, users who stack a ParserT on a StateT base monad will call the MonadState members directly like get instead of needing to do lift <<< get.

To get the ParserT internal state, call getParserT instead of get.

---

Checklist:

• Added the change to the changelog's "Unreleased" section with a link to this PR and your username
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation in the README and/or documentation directory
• Added a test for the contribution (if applicable)