Crank up the warnings and resolve as many as possible

[jpco]

Tested with gcc and clang on linux. I believe gcc's -Wclobbered is genuinely buggy and most or all of the warnings it produces here are false positives, so I'm leaving that one turned off, even though it seems handy for es' setjmp-heavy code. (See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65041 and attached issues for more on GCC's behavior.) Helps move toward #3.

[jpco]

It would be valuable to add -Wconversion as well, but fixing all the warnings caused by -Wconversion is a whole task in itself (and, I suspect, causes a dizzying diff).