Null terminate public strings

[mratsim]

This ensures that public strings are null-terminated so that they can be passed as-is to raw C routines.
It fixes #8

In terms of performance we have for request.data.toString(request.url.s, request.url.e):

• allocation of a string of length L (+1 for null byte)
• copyMem of a slice of length L

The original code cast[string](request.data[request.url.s..request.url.e]) had

• allocation of a seq[byte] of length L
• copyMem of a byte slice of length L
• reinterpreting the memory

So we should be as efficient.