getting started estimating the size of JSON string's Jil produces
first hack at actually sizing the StringWriter properly
failing test for autosizing (the estimator isn't great at the moment)
this action-ish approach isn't going to work, mapping back to expected size is too finicky; stopgap commit before re-implementing
a capacity estimator that doesn't suck
capacity estimation hooked up properly, some tests.  I default this off as while it appears to work when the estimates are correct, the estimates may not be correct enough often enough to be worthwhile.  There are some other approaches to fast TextWriters that may be worth pursuing in preference to capacity estimation.