Use a simple list for string code table

Use a simple list for string code table

Since the codes are assigned in a linear fashion, we can use
a simple ArrayList to track assignes codes, preventing the need
for boxing integer during population and lookups. Lookups also
end up requiring an array offset lookup rather than needing
to go through multiple indirections.

While this can in theory lead to a large array being allocated,
the set of strings we keep in the table is limited by the
SchemaContext and thus will be typically capped ad couple of
thousand.

The cost of growing the lookup should not be significantly
higher, even if ArrayList grows slower:
- 10, 16, 25, 38,  58,  88
versus HashMap (with .75 load factor)
- 12, 24, 48, 96, 192, 384

each resize operation is significantly faster.

Change-Id: I00c7a2e0985dbf901b6857a16424bf157e5af39d
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>