Add informative error messages when parsing command line options returns an error
The background behind why I'm submitting this PR is slightly embarrassing. After running `./mach` with some servo options I got the following stack backtrace.
```
thread '<main>' panicked at 'called `Result::unwrap()` on an `Err` value: ParseIntError { kind: InvalidDigit }', ../src/libcore/result.rs:746
stack backtrace:
1: 0x56459001b0b0 - sys::backtrace::tracing:👿:write::hb120982054a416e35nu
2: 0x56459001e02b - panicking::default_handler::_$u7b$$u7b$closure$u7d$$u7d$::closure.42840
3: 0x56459001dc96 - panicking::default_handler::h7ae2e4523ef4c187IFy
4: 0x56459000760c - sys_common::unwind::begin_unwind_inner::h5fb19481d14902dbDgt
5: 0x564590007cf8 - sys_common::unwind::begin_unwind_fmt::hd62d57279546b0f2Jft
6: 0x56459001a701 - rust_begin_unwind
7: 0x56459004c01f - panicking::panic_fmt::h27f7225e08792f40qYL
8: 0x56458f393901 - result::unwrap_failed::h18215968003310890981
at ../src/libcore/macros.rs:29
9: 0x56458f3937b9 - result::Result<T, E>::unwrap::h10319050269194353824
at ../src/libcore/result.rs:687
10: 0x56458f383701 - opts::from_cmdline_args::h8ea8d8c87dc6ee726lg
at /home/drobertson/git/servo/components/util/opts.rs:599
11: 0x56458d1f8cdf - main::h8517eb49d15e90fbNaa
at /home/drobertson/git/servo/components/servo/main.rs:53
12: 0x56459001d8f4 - sys_common::unwind::try::try_fn::h9076850075504893162
13: 0x56459001a68b - __rust_try
14: 0x56459001d36e - rt::lang_start::h58a22f304b0c1e19Oxy
15: 0x56458d2f4aa9 - main
16: 0x7f370496360f - __libc_start_main
17: 0x56458d1c04a8 - _start
18: 0x0 - <unknown>
Servo exited with return value 101
```
I immediately opened up `gdb` and got to work only to realize that the error was due to a typo in an option that had been `unwrap`ed at [components/util/opts.rs:599](https://github.com/servo/servo/blob/master/components/util/opts.rs#L599). Perhaps some more informative error messages will help prevent some future face-palm moments like mine 😄
I couldn't think of a good way to add tests for this. Please let me know if I missed something. Comments and critiques are welcome!
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This commit refactors the style crate to be completely independent of
the actual implementation and pseudo-elements supported.
This also adds a gecko backend which introduces parsing for the
anonymous box pseudo-elements[1], although there's still no way of
querying them.
https://mxr.mozilla.org/mozilla-central/source/layout/style/nsCSSAnonBoxList.h
Win32 support
r? @frewsxcv for python stuff
r? @pcwalton for the "remove usage of Gaol" stuff for Win32
r? anybody else for misc cargo.lock updates, etc.
This replaces #7878.
This works best with https://github.com/servo/mozjs/pull/71, too, to enable static linking, but can be run without (via some PATH hackery).
The instructions are here, and will be added to a .md file in the repo once the mozjs changes also land:
https://hackpad.com/Servo-on-Windows-C1LPcI2bP25
I'd like to get these changes landed because I've been rebasing them for months, they're otherwise quite stable, and don't affect our other platforms and targets.
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