Although it's perfectly valid to put a function pointer in
a capsule, as we've been doing since the start of rust/hg-cpython,
an array of function pointers has several advantages:
- it can hold several functions. That's our main motivation here. We plan to expose index_length() and index_node(), which will be needed for a Rust implementation of nodemap.
- it could also have data
- (probably minor in the case of Mercurial) proper support for architectures for which data and code pointers don't have the same size.