export native string functions in an internal namespace

to allow extending pixie.string using code written in pixie. (coming
soon.)
add pixie.string ns back again

for now just reexporting the fns from pixie.string.internal, but now
extendable from pixie.
allow 0 as indices to index-of in all arities
implement replace and replace-first in pixie.string
add pixie.string/join

please welcome test #101, btw. :sparkles:
Merge pull request #100 from heyLu/more-strings

More string functions