doTick: use exception-safe modifyMVar_ instead of takeMVar/putMVar

[ahihi]

fixes #1099, maybe.

disclaimer: i dont fully understand the logic of manipulating stateMV in doTick (and the other locations that touch this MVar)... at the beginning the value is taken out, and if everything runs successfully, a new value is put in. the exception handler doesnt touch stateMV, so it seems like any exception should leave it empty. yet i dont see the hang with all mini-notation syntax errors - for example, Tidal recovers just fine from d1 $ n "<0 1 2§>*16" # s "808". not sure how.

but i am making a guess that the intent here is not to leave stateMV empty on error. to this end, i replaced the takeMVar/putMVar sequence with a modifyMVar_. from my testing so far, this seems to resolve the issue, but im not sure if there might be some undesired side effects.

(i also moved the exception handler into its own function, as i found the previous parenthesized multi-line blocks difficult to read, but let me know if that style is preferred)

[yaxu]

Thanks, lets merge for now and try it out!