Cigar parsing speedup

[bkille]

According to perf, a good portion of runtime was spent converting slices of str to i32 in the parse_cigar_to_delta function.

Note that this PR gets rid of the check which makes sure that the number parses correctly (num_buf.parse::<i32>().map_err(|_| ParseErr::InvalidCigarFormat)?). As we are already checking if each character is a digit, I don't think there's any way that num_buf would throw anyways (unless parse throws if there are leading zeros, I am not sure...).

Benchmarks
Input:

> du -sh $PAF
78G     test-data/primates16.20231205_wfmash-v0.12.5.paf

Command (the .impg index was precomputed).

./target/release/impg partition --paf-file $PAF --sequence-prefix chm13#1 --window-size 1000000 -t 1

Main branch:

User time (seconds): 1081.02
System time (seconds): 61.03
Percent of CPU this job got: 97%
Elapsed (wall clock) time (h:mm:ss or m:ss): 19:29.24

This PR:

User time (seconds): 598.06
System time (seconds): 46.71
Percent of CPU this job got: 95%
Elapsed (wall clock) time (h:mm:ss or m:ss): 11:12.87