zaml: statically determine the to_ascii8bit implementation.

Instead of asking every single string the same question, "do you respond to
encoding", we can just figure that out once and choose an implementation
based on the result - since it is a static test on the same class every time.

This saves ~ 0.1 second of runtime, which is small but cheap.

Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>