Adopt core naming conventions for bigint methods

[tarcieri]

See also: #537

The upcoming bigint_helper_methods feature of core defines method names and type signatures which it would be good to adopt here as well where it makes sense.

Here's a name bikeshedding thread, if anyone has any opinions on these names: https://internals.rust-lang.org/t/naming-bigint-helper-methods-bike-shedding/21688