Symbolization is a costly operation: do it only when requested. Rework
the C API to make this possible. For now, do it automatically in the
Java API to avoid breaking existing Java programs: this will fix the
performance situation in the other C-based APIs.
Transition all relevant testcases to it. In the process, port
docstrings/comments from test.py files to expected_concrete_syntax.lkt
(now test.lkt) sources and fix stylechecks issues there.
This makes the convention consistent with the DSL, and avoids
workarounds for conflicts with Lkt keywords: in Libadalang, the Null
token can stay Null, instead of null_tok (no API breakage needed).
