Add alternative enum assigned name mapping

Add alternative enum assigned name mapping

This patch corrects enumeration mapping rules so that any unicode
string can be mapped to a valid Java identifier as per JLS8.

This is done by attempting to apply className() mapping to all names
and using that if it results in valid non-conflicting identifiers.

If a conflict is detected, we use a bijective mapping, which encodes
any conflicting characters using an $-based escaping scheme.

JIRA: MDSAL-309
Change-Id: Ic51405e533eff9af6afc2abaa8d0cc193d718e64
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>