mut keyword for declarations

[sunjay]

Though it won't be implemented until after MVP, it would be a breaking change to add the mut keyword to the syntax. To avoid that, add the keyword now and just leave it unenforced until that can be implemented.

• Add mut to grammar
• Update examples
• Document that mut is unenforced for the time-being but will get statically checked soon (change log is sufficient)

Static checking will be added in #64.