Adds custom_const_pointer_type and custom_mut_pointer_type options.

[remexre]

These options allow specifying an alternate type to use where pointer types would otherwise be used. This is useful for binding to a WebAssembly guest, as well as for certain kinds of serialization and deserialization.

Resolves #2023.

r? @emilio

[bors-servo]

:umbrella: The latest upstream changes (presumably b5a6813c9a6feaeda38d7c672a1ed155f9e27266) made this pull request unmergeable. Please resolve the merge conflicts.