Rubyfication

[rajithv]

For better intuitiveness for Ruby users, some SymEngine constructs can be changed.

@zverok Let's add such instances to a list here.

• Basic#to_proc instead of SymEngine::lambdify(basic)
• Map SymEngine::Symbol to Ruby Symbols. ( x = SymEngine(:x) )

[zverok]

About first, I still think the name should be to_proc.

lambdify could be an alias (for those knowing SymEngine better than Ruby), but for this case (matching Python API?..) it should be left as SymEngine::lambdify. So, what I propose here is:

• leave SymEngine::lambdify but explicitly state that Basic#to_proc is preferred;
• make Basic#to_proc to be able to do everything SymEngine::lambdify could do.

WDYT?

[rajithv]

Yep. That's better. Updated the checklist to reflect that.