Export declared regexes to generated code

Export declared regexes to generated code

This patch exports declared pattern regular expressions to binding
generators, thus allowing them to report violations using the declared
string rather than the mangled Pattern string we use to implement
enforcement.

As it turns out there is place for improvement with generated code
size, as decoding pattern negation requires a bit of repetition
as well as needing to capture additional strings. Since we can take
advantage of common code library, do not encapsulate single expressions
in an array.

This patch battles those ill effects by exposing CodeHelpers class
from yang-binding, which hosts common code blocks for reuse by
generated code. This means classes generated by this and newer codegen
plugins require a matching or newer version of yang-binding at runtime.

JIRA: MDSAL-313
Change-Id: I8abb33660c5182703f2db5faf0eee7e08ce15f06
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>