Change: removed byte strings API.

Change: removed byte strings API.

These functions are unsafe because they produce byte strings.
Byte strings may not work as expected with the existing JS methods.

The following functions were removed:
    - njs_vm_value_string_set() use njs_vm_value_string_create() as a
    drop-in replacement.
    - njs_vm_value_string_alloc() use njs_chb_t and
    njs_vm_value_string_create_chb() instead.

This fixes #710 on Github.