Replaces map-based system symbol table with hard coded implementation

[popematt]

Issue #, if available:

Fixes #34

Description of changes:

I was looking at a profile for a specific application and found that interning symbols was a non-trivial portion of the profile.

When testing the change with the ion-java-benchmark-cli, I was seeing a 5-10% improvement in writing Ion Binary for datasets with very high proportions of symbols (real world example—very wide structs). Results are inconsistent (maybe because the JVM Hotspot optimizations are not entirely deterministic or because of GC delays 🤷‍♂️), but have never indicated that this change makes performance worse.

However, I have run into an issue where if SharedSymbolTable. ION_1_0_SYSTEM_SYMTAB uses this implementation, the test suite does not pass. It seems like the old implementations of the system symbol table are inconsistent depending on whether you get it from Symbols or SharedSymbolTable. After discovering that, I realized I need to step away from this for a bit in order to get other things done. I'm not sure when/if I will continue this.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.