Add 'katipSetContext' which doesn't leak

[jappeace]

Fixes #153

This introduces an alternative api which allows the following code to run without memory build up.

logLoop :: KatipContextT IO ()
logLoop = do
  katipSetContext "xx" someContext $ do
      $(logTM) InfoS "Hello Katip"
      logLoop

new katipSetContext:

compared to katipAddContext:

NB, see my reproducer repo for the code that generated these graphs:
https://github.com/jappeace/katip

My initial attempt failed because the map needed to have a strict field (I'm uncertain why).

[ozataman]

Taking a stab at this without running the code (sorry not able to ATM). The way you're looping, aren't you just repeatedly calling katipAddContext in each loop? So you keep adding/setting context in deeper calls. I assume that you're accumulating large thunks in the data type underlying the sl construction. Instead, you should be a setting the context a single time, then loop the inner part of the code.

[jappeace]

btw those recent changes made this work ^ @MichaelXavier @ozataman