MIR: Add a minimal parser

Context

The changes here contains a parser that can parse a minimal subset of Michelson, just enough to process the Fibonacci implementation from this MR.

The parser only returns a boolean to indicate a valid syntax. There are two tests included, one that expect a successful parse of the Fibonacci listing, and the other that fails on a slightly broken version of the same.

Manually testing the MR

Please use cargo test command to run these tests.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR