tokenize() too slow

[soimort]

This mainly affects theme loading, and parsing of large JSON data.

When the user has a large theme file (> 2k bytes), trans suffers from slow startup time.

With a theme:

0.43s user 0.03s system 91% cpu 0.503 total

Without a theme:

0.11s user 0.04s system 78% cpu 0.182 total