Feature request: no-unsed-functions

[gitpusha]

Requesting new rule to be added to solhint that's similar to no-unused-vars but for no-unsed-functions and it would look at all functions that are not virtual and thus should really be used.

[dbale-altoros]

@gitpusha As I see it, we should only apply this rule to private functions on the same contract since solhint only works at file level If a function is in contract A and another contract (B) is calling A's function, when solhint evaluates A it will show that the function is not being called...