Future API changes

[ljedrz]

With the latest PR I've noticed one potential improvement that is unlikely to ever be truly needed, but would make the library more airtight; unfortunately it would break the API, so it will need to wait for a 4.0 release. Therefore, I'm opening this issue to list any such potential future changes, and I'll be collecting them into a dedicated branch.

Breaking changes (a checkmark indicates that the 4.0 branch already contains the item):

• return usize from Term::max_depth
• check the API guidelines (this library is quite old by now 👴); I'm sure at least C-CONV is missing
• possibly const-ify some of the functions
• is_isomorphic_to should be named is_alpha_equivalent #59