Added static and const where appropriate.

Marked internal functions as static not to export them and for compiler
to be able to optimize them more aggressively.
Placed data tables in .rodata section by declaring them static const.
Avoid explicit extern C while including ssw.h
before merge ahippo's change
Merge branch 'master' of https://github.com/ahippo/Complete-Striped-Smith-Waterman-Library

some functions and data are announced as static