syntax: modernise attribute handling in syntax::attr.

This does a number of things, but especially dramatically reduce the
number of allocations performed for operations involving attributes/
meta items:

- Converts ast::meta_item & ast::attribute and other associated enums
  to CamelCase.
- Converts several standalone functions in syntax::attr into methods,
  defined on two traits AttrMetaMethods & AttributeMethods. The former
  is common to both MetaItem and Attribute since the latter is a thin
  wrapper around the former.
- Deletes functions that are unnecessary due to iterators.
- Converts other standalone functions to use iterators and the generic
  AttrMetaMethods rather than allocating a lot of new vectors (e.g. the
  old code would have to allocate a new vector to use functions that
  operated on &[meta_item] on &[attribute].)
- Moves the core algorithm of the #[cfg] matching to syntax::attr,
  similar to find_inline_attr and find_linkage_metas.

This doesn't have much of an effect on the speed of #[cfg] stripping,
despite hugely reducing the number of allocations performed; presumably
most of the time is spent in the ast folder rather than doing attribute
checks.