make contribs independant of the directory structure of picrin
remove miscommited test
Revert "make contribs independant of the directory structure of picrin"

This reverts commit cdee9d91349ba53a23244d1f3ea5061c8c074975.
mv FindREGEXP.cmake to under 10.regexp
Merge branch 'master' into refactor-contrib
Merge branch 'master' into refactor-contrib
Merge branch 'master' into refactor-contrib
add custom target `test-contrib` and make `make test` runs that
add test for regexp
add test for `for`
fix the test cases of `regexp-replace`
add test case for `null` of (picrin control for)
update xhash
use brand new API style of xhash
pic_sym should be an alias to int (for the convenience to use xhash for pic_sym)
Merge branch 'update-xhash'
compiler may perform tco against macroexpand_node
remove unused helper function
remove old code that has been commented out for a long
small refactor