Feature/255 keep fields ordered marshal struct

[brendesp]

Here is a fix for #255

Tracking the order of members as they are marshaled so they can be written in order of the structure. Default behavior stays the same.

New func MarshalOrdered() allows a marshalOrder to be passed in. Current options are OrderAlphabetical (default) and OrderPreserve

[codecov]

Codecov Report

Merging #266 into master will increase coverage by 1.25%.
The diff coverage is 93.33%.

@@            Coverage Diff             @@
##           master     #266      +/-   ##
==========================================
+ Coverage   90.72%   91.97%   +1.25%     
==========================================
  Files           9        9              
  Lines        1789     1845      +56     
==========================================
+ Hits         1623     1697      +74     
+ Misses        120      103      -17     
+ Partials       46       45       -1

Impacted Files	Coverage Δ
marshal.go	90.61% <100%> (+0.21%)	⬆️
toml.go	85.22% <85.71%> (+6.84%)	⬆️
tomltree_write.go	85.09% <93.6%> (+4.7%)	⬆️

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[pelletier]

Thank you so much for tackling that issue! I know it will make a lot of people happy.

Overall the code looks good. I think writeToOrdered will make it easier to fix #216.

However, I don't think introducing a MarshalOrdered function is a good interface. There is already a way to tweak the marshaling operation: the Encoder. I am thinking something like:

var buf bytes.Buffer
err := toml.NewEncoder(&buf).Order(toml.OrderPreserve).Encode(v)

A bit more verbose, but reuses the existing structure and allows for more flexibility, while keeping the API as small as possible. Definitely open to suggestions though!

Thank you again.

[brendesp]

I implemented you suggestions.

[pelletier]

Looking great! Thank you!