Cache rb_intern/symbol lookups in the lexer.

For JRuby this has little to no benefits as it uses strings for method names.
However, both MRI and Rubinius will perform a Symbol lookup whenever rb_intern()
is called. By doing this once for all callback names and caching the resulting
VALUE objects the lexer timings can be reduced by about 25%. In case of the
benchmark benchmark/xml/lexer/string_average_bench.rb this means it runs in
around 500ms instead of 700ms.