square braces can cause exponential time/memory consumption

[dwrensha]

The following input (found via fuzz testing) causes lean to take 140 seconds and consume 11.6 GB:

def foo : [[[[[[[[[[[[[

Adding more braces seems to make it exponentially worse.
My expectation is that, even if lean must perform exponential work in some situation, some kind of heartbeat will trigger before things get out of hand like this.

Lean (version 4.0.0-nightly-2022-10-18, commit faa612e7b79a, Release)

[leodemoura]

Plan: add cache to parser, no choice when there are errors