string: improve performance of at and ord

do not calc unneccessary utf8_strlen
do not recalc unneccessary utf8 offsets
add potion_utf8char_nextchar for str_at (gets fast length of next utf8 char)
potion_str_at: need signed start
example/ackermann.pn added. crashes
fix example/ackermann.pn

arithmetic is whitespace sensitive: m-1. fixme
fix whitespace in example/nqueens.pn also

but this is still broken
whitespace only
libuv-1.0: readdir->scandir, add scandir_next

also add mkdtemp, FS_MKDTEMP, FS_ACCESS, DIRENT types
numeric cmp in bytecode #76
numcmp jit

Note that SSE cmp via ucomisd needs other jump ops, than non-SSE cmp:
iop: jl, jg, jle, jge, je, jne for normal cmp comparisons,
xop: jb, jbe, jae, ja, je, jne for SSE ucomisd comparisons.

TODO: optimize true/false with less jumps
travis: add osx (beta)
3rd/libuv: remove defunct --disable-dtrace, update 1.0 conf fix

deleted upstream
add -msse2 on intel, pentium 4, for numcmp
Merge remote-tracking branch 'potion/master' into p2
update submodule libuv to master