Allow specifying whitespace on save

[cmyr]

This lets the user optionally customize the whitespace used during
serialization.

The implementation is slightly annoying. the quick_xml and rust-plist
crates specify whitespace in different ways; quick_xml takes a (u8,
usize) pair (the byte and its count) and rust-plist takes a
Cow<'static str>. Additionally, in our own glyph serialization code we
need to write whitespace directly when appending the 'lib' section.

Anyway: you create a WriteOptions object by passing it a string, and
then we figure out what the appropriate (u8, usize) pair is. The reason
we want to think about this is because eventually writing out a UFO
should be parallelized, and that could mean cloning WriteOptions once
for each glyph; we would like to make sure this isn't allocating.

So this seems to work, even if some of the internals are a bit gross.

---

this is based off of #147, which should go in first.

[github-actions]

🗜 Bloat check ⚖️

Comparing 7c97df8 against f1a82f2

target	old size	new size	difference
target/release/examples/load_save	1.76 MB	1.75 MB	-15.52 KB (-0.86%)
target/debug/examples/load_save	8.17 MB	8.17 MB	-776 Bytes (-0.01%)

[chrissimpkins]

@cmyr are you actively working on XML declaration single quote support? If not, I will have a shot at it based on the changes in this PR

[cmyr]

@chrissimpkins I took a peek haven't implemented anything, but let's merge this first, so we can use WriteOptions.

I also think that it's worth being slightly fancier and doing the replacement before the initial write, it just feels more principled, and will be more portable (the write-then-rewrite approach may be slower on older storage media, or on other platforms.)

[cmyr]

@chrissimpkins to elaborate on what I would propose:

• add a quote_char method to WriteOptions
• When serializing the .glifs, (which we serialize manually) write the header to the stream ourselves, using the specified character
• When serializing plists, we don't control writing the header, so we'll have to rewrite. There are options here. All plist writing is now handled through a method in write.rs.
  • One option is to write to disk, and then use the unix/windows specific methods to go and overwrite the header in place, before closing the file handle: see std::os::unix::fs::FileExt and std::os::windows::fs::FileExt; you'll need slightly different impls on either platform.
  • the other option would be to have a custom Write type, that would wrap or replace the BufWriter; this would intercept the header and replace it, if needed, otherwise forwarding the bytes to the inner writer.

Thinking about this, I think the first option is totally reasonable. The second option is probably marginally more 'efficient', but I don't think it matters; the main thing we want to avoid is closing and reopening all the file handles.

edit: broke this out into an issue, #156