improve BAM.quality performance

[jonathanBieler]

This removes the reinterpret and type assert from BAM.quality and instead returns a vector with the raw qualities from record.data, which improves performance a bit, specially when it comes to memory allocations:

Computing the average quality on 5M reads

Master:  17.907719 seconds (20.33 M allocations: 2.483 GiB, 3.22% gc time)
This PR: 16.474836 seconds (5.33  M allocations: 1.109 GiB, 1.03% gc time)

I've tested the change on bams produced by different aligners (bwa, bowtie & hisat2) and the retrieved qualities were identical. Fix #32

[codecov]

Codecov Report

Merging #33 into master will increase coverage by 0.45%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
+ Coverage   56.82%   57.28%   +0.45%     
==========================================
  Files          27       27              
  Lines        1883     1868      -15     
==========================================
  Hits         1070     1070              
+ Misses        813      798      -15

Impacted Files	Coverage Δ
src/bam/record.jl	36.45% <33.33%> (+1.74%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update cb5427a...a35981a. Read the comment docs.