some optimizations: compute the unicode final length during newline translation

instead of calling check_utf8 a second time

--HG--
branch : py3.7
2 jobs for branch/py3.7 in 8 minutes and 50 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #195147
check_rpython_annotations

00:07:09

passed #195148
hpy_tests

00:08:50