Use intern rather than symbolize it Puppet::Type

The cost of symbolize is ~ 50 percent higher than the cost of intern, when
applied to either a string or a symbol. Given this is called some 90,000
times in my real world test run, that adds up to a non-zero amount of
wasted time.

Sadly, this only works out at 0.15 seconds real time reduction, but it is
better than nothing - and takes out 85,000 extra calls.

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