Avoid -Wcast-function-type warnings

[Enchufa2]

Closes #1500. Addressing this one first because this flag is added by -Wextra and it gets in the way of other warnings.

As the gcc manual suggests, and others apply, I've defined a new typedef void (*RCPP_FUNC)(void) that is used as a generic intermediate cast. This affects not only our function casts, but also to the generated interfaces, so client packages will see this warning go away too only when they regenerate their interface with an updated version of Rcpp.

Checklist

• Code compiles correctly
• R CMD check still passes all tests
• Preferably, new tests were added which fail without the change
• Document the changes by file in ChangeLog

[eddelbuettel]

Very nice!