The dodgy stack of XML handlers we had in C is gone now; it's replaced
with a much simpler scheme in Rust code.

Pending things:

- Actually call libxml2 from Rust

- Handle XML entities in Rust

- Handle XML processing instructions in Rust

- Move the rsvg_handle_acquire_() machinery to Rust; this is basically
  rsvg-io.c and the allow_load() stuff.