`definition_for` is used to extract the line numbers of a test, which
is relevant when filtering tests to a line number or range, eg
my_test.rb:1-2.

The filter itself operates by, for each test, comparing the lines the
test spans against the filter. In the current code, this will call
`Prism.parse_file` and then walk the parse result until the matching
definition/call is found.

For files with a large number of tests, this is fairly onerous and can
dominate the test runtime. Fortunately, we can parse the file once and
cache the starts and ends of every def/call node.
This beings the `definition_for` runtime down from seconds to low
milliseconds.

We also see a reduction in gc time that is probably attributable to
not repeatedly parsing the file.
For a test run in an 240 test file that went from 11s of parsing to
20ms, gc went from 4.5-2.2 seconds.

Finally, for any user that has written a reporter or otherwise hooked
into the filters, this should provide speedup as well.

Co-authored-by: Stath Stivaros <stath.stivaros@shopify.com>