Let compiler synthesize Equatable conformance for _XMLElement

[jsbean]

As _XMLElement was made a struct in 1f0cb9d, we can let the compiler synthesize Equatable conformance for free.

This PR removes the custom _XMLElement: Equatable conformance implementation.

[jsbean]

Ah, so SwiftFormat does not like the empty : Equatable { } notation. Otherwise, the tests pass.

Would you prefer the conformance to be placed up top (struct _XMLElement: Equatable {) or in some other fashion?

It looks like I misunderstood the rule (emptyBraces). Should be acceptable as of 0d75c10.

[codecov]

Codecov Report

Merging #33 into master will decrease coverage by 0.24%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
- Coverage   51.57%   51.32%   -0.25%     
==========================================
  Files          29       29              
  Lines        1400     1391       -9     
==========================================
- Hits          722      714       -8     
+ Misses        678      677       -1

Impacted Files	Coverage Δ
Sources/XMLCoder/Auxiliaries/XMLElement.swift	86.99% <ø> (-0.13%)	⬇️

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