Slight refactor, eventually

[patrick-kidger]

Not suggesting that this happen now; I'm just opening an issue to keep track of it.

I think the current hierarchy of:

_brownian
_core
    methods
brownian_lib
_brownian_lib [from C++]

(not counting _core/adjoint_sdes which I know you've removed in your latest branch)

should be switched to:

_impl
    brownian
    methods
    core

(So additionally tidying the Brownian things together somehow. If we end up implementing every Brownian* in C++ then that would suffice, for example.)

[lxuechen]

Agree. This refactoring seems sensible, and should come in when we have the C++ implementations.