base64-0.5.2

A point release on top of v0.5.1 to fix some bugs and shortcomings. Most important changes:

• #132: CI: add tests for MSYS2 environments
• #127: bin/base64: compile on Windows by emulating writev(2)
• #126: bin/base64: ignore newlines when decoding
• #125: portably detect word size, now works properly on Windows
• #6: benchmarks: compile on Windows