avro

AVRO-3305: Only mangle type identifier keywords when used as types  (#1457)

* AVRO-3305: Only mangle type identifier keywords when types

Instead of treating all Java keywords identically, only mangles what
the JLS terms as "contextual keywords" when used in the context they
are reserved. Specifically, only mangle reserved type identifiers
when used as type identifiers.

* AVRO-3305: Extend tests to test mangling of all types of keywords

Previously tests only tested with a subset of the keywords accounted for
in the Avro compiler. This commit extends those tests to test against all
known keywords.