XML encoder implementation

[Alma-media]

This PR introduces the ability to encode and decode XML objects.

[codecov-commenter]

Codecov Report

Merging #120 into master will decrease coverage by 0.60%.
The diff coverage is 78.49%.

@@            Coverage Diff             @@
##           master     #120      +/-   ##
==========================================
- Coverage   86.70%   86.10%   -0.61%     
==========================================
  Files          89       98       +9     
  Lines        5166     5425     +259     
==========================================
+ Hits         4479     4671     +192     
- Misses        426      474      +48     
- Partials      261      280      +19     

Impacted Files	Coverage Δ
pkg/codec/json/object.go	83.09% <ø> (+0.69%)	⬆️
pkg/codec/www-form-urlencoded/errors.go	100.00% <ø> (ø)
pkg/codec/www-form-urlencoded/types.go	100.00% <ø> (ø)
pkg/codec/xml/object.go	57.14% <57.14%> (ø)
pkg/codec/xml/xml.go	68.96% <68.96%> (ø)
pkg/codec/json/array.go	81.48% <81.48%> (ø)
pkg/codec/xml/array.go	86.66% <86.66%> (ø)
pkg/codec/xml/decode.go	89.09% <89.09%> (ø)
pkg/codec/xml/encode.go	94.28% <94.28%> (ø)
pkg/codec/www-form-urlencoded/form.go	92.06% <100.00%> (ø)
... and 12 more