nimony: add basic ${path} to {.header.} for relative c include

[mrgaturus]

• add replaceSubs helper to semos.nim to replace a pattern by an absolute path
• add basic ${path} and ${nifcache}support to {.header.} + some tests
• add a -I of root module path relative to nifcache in final makefile for nim 2.x backwards compatibility (see Add project directory to include path Nim#10012)

fixes #431

[Araq]

• Use relative paths if possible so that if we add the produced makefiles to the testsuite they can actually work.
• The signature should be replaceSubs*(f, currentFile, nifDir: string): string
• Use strutils.multiReplace in replaceSubs's implementation.

[mrgaturus]

• Use relative paths if possible so that if we add the produced makefiles to the testsuite they can actually work.
• The signature should be replaceSubs*(f, currentFile, nifDir: string): string
• Use strutils.multiReplace in replaceSubs's implementation.

I updated the ${path} and c include header thing to work as relative and updated the replaceSubs to support a ${nifcache}