Change mangling for leading hyphen

[scauligi]

As brought up in #1993, the single underscore is a frequent idiom in python and would shadow the subtraction operator.

I propose that leading (and trailing?) hyphens be mangled like other characters, and only inner hyphens be mangled to underscores.

This means that e.g. --init-- is no longer the same thing as __init__, but using hyphens for dunders (and for private names) was nominally against Hy's style guide anyway.

This also means that (import [module [*]]) can pick up on symbols like - or -> without having to use __all__, since they no longer have leading underscores.

[peaceamongworlds]

This seems to be the same as #1635

[Kodiologist]

I agree. That's the right issue for discussing changes to hyphen mangling. Its title can be changed to taste.