Implement heapsize for all the libraries servo uses

[notriddle]

These are all the libraries that had HeapSizeOf definitions in components/util:

• azure
• cssparser
• euclid
• html5ever
• hyper
• mime
• layers
• mozjs
• string_cache
• selectors
• std
• url
• util itself (range, cursor, geometry, logical_geomtry)

[jdm]

I am keen to add hyper support, too.

[Manishearth]

Should we advertise this library as something for the general community to use?

[SimonSapin]

@nnethercote suggests moving it into std so that std types can implement it in less hackish ways (e.g. getting a pointer from a private field): #1 (comment)

[Manishearth]

I'm not sure if that will happen -- even the regex lib (arguably more useful) is not part of std.

Post-MIR we may be able to generalize derive so it can be applied to crates preceding the one defining the trait via metadata (not sure if private fields will work).

[SimonSapin]

The argument is not that it’s more useful, but that implementing it for std types outside of std (so without access to private fields) requires guessing or crazy unsafe code.

[Manishearth]

Sure, but that just makes a case for why this should be in std, not for why std would want this 😄

[jdm]

std is the ideal here, but I'd like further experience making use of it in the wider world before committing to the RFC process.