Remove "extern" from function declarations in header files

[lrstewart]

Problem:

Putting "extern" in front of a function declaration is redundant-- it's implied. We only want "extern" in places like variable declarations and api/s2n.h function declarations.

Unless I'm missing something and there's a valid reason we need explicit "extern"s in front of our function declarations?

Proposed Solution:

Short term: clean up header files to remove redundant "extern"s.
Long term: some sort of linter / add it to simple mistakes script
If we do need "extern": document why