Give the parsing threads more stack, and quiet a recovered Prism panic

[dduugg]

Summary

Two fixes from a review of #73 after it merged. Neither problem has been released, since #73 didn't bump the version.

• Deeply nested files no longer abort pks. Prism parses recursively, as pks's walk of its tree does, so each level of nesting takes stack. On a rayon thread's default 2 MiB, about 1,400 to 2,500 levels of nested calls, hashes, arrays or blocks overflowed it, and the whole run aborted with fatal runtime error: stack overflow. lib-ruby-parser held 16,000 levels or more. The global thread pool's threads now get 16 MiB.
• No panic text before the non-UTF-8 warning. pks recovers from the panic that Diagnostic::message raises on a message that isn't UTF-8, but Rust's default panic hook printed it first, with a path into the cargo registry, so the run looked as if it had crashed. Now the warning is all that prints.

Design notes

• Why 16 MiB. Prism stops nesting after 10,000 levels and reports "nesting too deep". With 16 MiB, each of 14 constructs nested 12,000 deep parses with that warning. 8 MiB still overflows on arrays, parentheses, !, splats and indexes. Method chains and elsif chains are flat in Prism's grammar but deep in its tree, so the cap doesn't stop them, but they overflowed 2 MiB only past about 16,000 levels.
• Cost. A thread's stack is reserved, not committed, so peak memory on two large Rails apps is unchanged. The machine was busy while I measured, so I don't claim a timing result, though hyperfine showed no difference beyond its noise.
• The panic hook. ruby-prism has no way to read a message that can't panic, so pks still catches the panic. The hook it installs leaves out only a panic raised while a message is being read, on that thread, and passes every other panic to the hook it replaced. The proper fix belongs in ruby-prism, where Diagnostic::message could replace invalid bytes instead of panicking.
• The test's depth. A debug build takes more than ten times the stack per level. It overflows 2 MiB at about 200 levels of nested arrays and 16 MiB at about 1,300, so the test nests 500.

Test plan

• New test: list-references on 500 nested arrays, under both parsers. It aborts without the stack change.
• The non-UTF-8 test now requires stderr to be exactly the warning. It fails without the hook.
• cargo fmt --check, cargo clippy --all-targets --all-features -- -Dwarnings and the full suite pass.