Switch to bstr for escaping mostly UTF-8 binary data

[chenxiaolong]

bstr is Unicode-aware and won't split multi-byte codepoints into multiple \x##.

This commit also adds an escape module for use with serde so we can serialize mostly UTF-8 binary data into human-readable files, like TOML.

[chenxiaolong]

I'm going to switch to using the bstr crate for doing the escaping instead. It's Unicode-aware and encodes foobar你好<ff byte> as foobar你好\xff instead of foobar\xe4\xbd\xa0\xe5\xa5\xbd\xff. Since avbroot uses escaping to show (mostly UTF-8) binary data as human readable as possible, having Unicode support is good.