Add fast function for comparing attestation data

[0xKiwi]

What type of PR is this?
Addition

What does this PR do? Why is it needed?
This PR adds a function to help check a comparison between 2 attestation data fields, since its performed a lot within the slasher.

Benchmarks:

BenchmarkAttDataIsEqual/fast
BenchmarkAttDataIsEqual/fast-8          58908008                18.1 ns/op             0 B/op          0 allocs/op
BenchmarkAttDataIsEqual/slow
BenchmarkAttDataIsEqual/slow-8            442920              4363 ns/op             384 B/op         30 allocs/op

[codecov]

Codecov Report

Merging #5834 into master will decrease coverage by 0.14%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master    #5834      +/-   ##
==========================================
- Coverage   59.77%   59.63%   -0.15%     
==========================================
  Files         312      312              
  Lines       26680    26320     -360     
==========================================
- Hits        15949    15695     -254     
+ Misses       8570     8486      -84     
+ Partials     2161     2139      -22     

[shayzluf]

Thanks for doing it