Create mutating mechanism

[kurtlawrence]

Related to #22.

Scope

• Add mechanism to mutate app_data

Mechanism

• Add .mut command
  • Args afterwards can be sent straight to eval stage?
• Flag for mutating
  • Automatically close after eval?
• Change fn signature (app_data: &mut T)
• Once evaluated, further eval loops will not include the mutation code

The mutation has occurred with a single evaluation stage, such that it is unnecessary to repeat the same mutating code. Movement of data into the app_data will also not capture the data as a repeated eval loop will reconstruct that data. Assigned variables whilst in mutating state will not persist in further eval loops (the code is erased).

[kurtlawrence]

Not able to take args unfortunately, it would require the ability to keep passing through read data, which for now is not implemented.