Use a known normalization algorithm in `utils.hgvs._normalized_vcf` function

[erikyao]

There are a few VCF normalization tools/algorithms that can be used in our _normalized_vcf function, e.g.:

• bcftools
• GATK
• SMaSH
• vt
  • Ref Python Implementation

Our current implementation fails to meet the Parsimony definition of VCF normalization:

A variant is parsimonious if and only if it is represented in as few nucleotides as possible without an allele of length 0.

E.g. our current implementation cannot handle cases like ref = "TCCCCT", alt = "CCCCT". Normalized sequences should be ref = 'TC', alt = 'C'.