Fixes for 32-bit in stat and stream

Most important is the impact on filesize. Since libuv returns a 64-bit
value on all platforms, we might as well make use of it.
Otherwise 32-bit Julia builds can be needlessly confused about the size of
files.
Merge pull request #3616 from timholy/fix32bit

Fixes for 32-bit in stat and stream
Fix inv for triangular matrices
Update README.windows.md

Clarify python requirements
Fix #3622