src/profiler.c: Keep track of the discarded counts

src/profiler.c: Keep track of the discarded counts

When the table overflows and wh evict entries, keep track of those
counts in a global counter so we can see the proportion of
samples this represents.

* src/profiler.c (struct profiler_log): Add `discarded` field.
(evict_lower_half): Change arg to be `struct profiler_log`.
Transfer counts to the new `discarded` field.
(record_backtrace): Change arg to be `struct profiler_log`.
(add_sample): Adjust call accordingly.
(export_log): Add `discarded` counts to the result.
Onle add the GC and `discarded` counts if they're non-zero.
(syms_of_profiler): Define new symbol `Discarded Samples`.