Test to explore if re-using Parser instances is faster.

Tests on my laptop show:
- overhead by introducing the stack is in the noise for parsing even the
simplest expression
- efficiency from re-using the ELParser is measurable for even a single
reuse of the parser
- with large numbers of parses (~10k) performance for a trivial parse is
three times faster
- around the 100 iterations mark GC overhead adds significant noise to
the results - for consistent results you either need fewer parses to
avoid triggering GC or more parses so the GC effects are evenly
distributed between the runs

Note that the test is single threaded.