Add tests/netint_test.c and CMakeLists.txt changes to test rs_int_len().

In netint.[ch] replace runtime checks for invalid arguments with assert()
statements and tidy the argument names, variable names, and ordering. Replace
`unsigned char` arguments with `rs_byte_t`. Add an assert to rs_int_len() to
require the argument is positive.

Make code variables and structure more consistent. Add extra assert() check to
rs_tube_copy_from_scoop(). Replace fatal runtime check with assert() in
rs_tube_write().