gfx: Micro-optimize `DList` and `StackingContext` a bit.

I'm sad to say that this improved performance significantly. A lot of
this win is due to the Rust compiler not being smart about not zeroing
objects out if it doesn't need to.
Add serve-docs command to mach

This enables the user to run `./mach serve-docs` (with an optional port
number), starting a local web server that hosts the documentation for
Servo and Rust.

This closes #3807
Defer to GetAttribute in HasAttribute.

The semantics of has_attribute aren't anywhere close to the ones expected for
Element#hasAttribute, and it fails an assertion when passed non-lower-case
names.
Re-enable reflection-forms.html.
Pass an Atom to VirtualMethods::parse_plain_attribute.
auto merge of #3890 : pcwalton/servo/microoptimize-dlist, r=glennw

I'm sad to say that this improved performance significantly. A lot of
this win is due to the Rust compiler not being smart about not zeroing
objects out if it doesn't need to.

r? @glennw
auto merge of #3896 : Ms2ger/servo/attr-crash, r=jdm

The semantics of has_attribute aren't anywhere close to the ones expected for
Element#hasAttribute, and it fails an assertion when passed non-lower-case
names.
Parse the size attribute on input elements.
auto merge of #3897 : Ms2ger/servo/enable-reflection, r=Manishearth
Only print HTML parse errors with RUST_LOG=script::parse
Fixes #3353: rust snapshot downloads to home directory
Add documentation to plugins crate
auto merge of #3900 : kmcallister/servo/parse-error, r=Ms2ger

r? @ms2ger
Pass the correct argument to Element::create when cloning.

The current code calls as_slice() on the Option, yielding &[DOMString], and
then calls to_string, yielding "[prefix]".
auto merge of #3898 : Ms2ger/servo/prefix-clone, r=jdm

The current code calls as_slice() on the Option, yielding &[DOMString], and
then calls to_string, yielding "[prefix]".
Catch all types of local bindings in check_stmt
Implement HTMLElement.title.
Panic when bootstrap downloads fail

Also hints that 32bit snapshots are unavailable on failure

Resolves #3739

fixup! Panic when bootstrap downloads fail
auto merge of #3901 : Ms2ger/servo/title, r=jdm
Automatically update cargo when necessary