Component handling: add API for checked insertion and removal?

[madig]

I had the idea of having Layer and Glyph cooperate to check that inserting, modifying and removing components ensures that we never end up with dangling components. glifparser has code for handling. Need to investigate:

1. Does it add value?
2. Should it be used for UFO loading or just for the programmatic API? When loading UFOs, how to handle glyphs that exist but aren't added yet?
3. Are there situations where you'd need an unchecked way of messing with components?

[cmyr]

this will actually be kind of annoying, since when you add a component you're mutating the glyph directly, unless we have a method on Layer, like,

fn add_component(&mut self, component: Component, to_glyph: GlyphName) -> Result<(), SomeError> { }

in which case the layer could check that the referenced component exists in that layer?

In any case, I don't think this is pressing enough that we should block 1.0.

[madig]

Hm... not a blocker, no. I was thinking of Glyph::add_component_checked(&mut self, component: Component, layer: &Layer) but then you could pass in the wrong layer. Needs more thinking (and checking what MFEK stuff is doing).