(most of this by rillig@)
o lots of style and white-space cleanup
o lots more unit tests for variable modifiers
o simplified description of some functions
o str.c: refactor Str_Match
o var.c: debugging output for :@
  constify VarModify parameter
  fix :hash modifier on 16-bit platforms
  remove unnecessary forward declarations
  refactor ApplyModifier_SysV to have less indentation
  simplify code for :E and :R
  clean up code for :H and :T
  refactor ApplyModifiers

* var.c: we need stdint.h on some platforms to get uint32_t
* unit-test/Makefile: we need to supress the specific error
for RE substitution error in modmisc, since it varies accross
different OS.