Add an extra test for StateT's Apply instance

Catches a potential subtle bug with wrong sequencing of effects.
Add basic tests for 'start' and 'end' functions
Fix wrong comments
Merge pull request #62 from djv/patch-1

Add an extra test for StateT's Apply instance
Merge pull request #64 from djv/patch-3

Fix wrong comments
Added test cases for left apply, right apply. Fixes #65.
Merge pull request #63 from djv/patch-2

Add basic tests for 'start' and 'end' functions
Remove non-total warnings, as per #63.
Merge remote-tracking branch 'upstream/master'
void impl