Fixup load api

[cmyr]

This is mainly intended to clear up confusion around how to use
load_ufo; the answer is "don't".

I believe that the previous API of splitting up the creation of the font
from the loading was unnecessary. I also suspect that having the Font
object hold on to the DataRequest is unnecessary, although there are
possible arguments in favor of this approach.

In any case, the main confusion with the previous API is that loading
with a data request took &self by reference but returned a new font
object, which is confusing. I would expect it either to take a
DataRequest as an argument to an associated function, or I would expect
it to take &mut self and load the contents of the file into the
current font?

@ctrlcctrlv Does this change work for you? Do you ever do anything where you create a Font and then do some other stuff before calling load_ufo? My medium-term vision would be to maybe remove the data_request field on Font; alternatively we could keep it, but only exposed immutably (via a getter?) my thinking is that mutating the request after the font is loaded doesn't do anything, but could confuse the user?

closes #138

[github-actions]

🗜 Bloat check ⚖️

Comparing ae135ea against 6f64b48

target	old size	new size	difference
target/release/examples/open_ufo	1.69 MB	1.67 MB	-18.74 KB (-1.08%)
target/debug/examples/open_ufo	7.05 MB	7.05 MB	-1.84 KB (-0.03%)

[github-actions]

🗜 Bloat check ⚖️

Comparing bfbbc1c against 07324c0

target	old size	new size	difference
target/release/examples/open_ufo	1.69 MB	1.67 MB	-18.7 KB (-1.08%)
target/debug/examples/open_ufo	7.05 MB	7.05 MB	-1.92 KB (-0.03%)

[madig]

I don't think we need to keep request around, except when we eventually (?) implement lazy loading?

[madig]

@cmyr How bout making this

pub fn load(path: &dyn AsRef<Path>) -> Result<Font, Error> {
    Self::load_requested_data(path, DataRequest::default())
}

pub fn load_requested_data(
    path: &dyn AsRef<Path>,
    request: DataRequest,
) -> Result<Font, Error> {
    // ...
}

Or something so we don't have to have an inner impl function to aoid monomorphization?

[cmyr]

Request was added in #53 because it was needed by @ctrlcctrlv, so I don't anticipate removing it.

I'd be happy to look at various patches to remove that inner function and see how they look?

[cmyr]

I'm going to merge this as-is, and if fred has any comments we can always revise it later.