Add *Tree.SetPositionPath

[sclevine]

I needed to add this function to support generating arbitrary, ordered TOML from other data serialization formats in yj.

*Tree.SetPath does not set Positions correctly when other *Trees are used as values. This makes the Encoder behave strangely when OrderPreserve is set: https://play.golang.org/p/Lns42AVkEcT

Additionally, *Tree.SetPath doesn't appear to be sufficient to generate arbitrary TOML when *Tree values are not used. For example, there doesn't appear to be a way to append tables to a list.

Adding this method seems to be all that's necessary to allow arbitrary, ordered TOML to be generated, which seems useful.

[codecov]

Codecov Report

Merging #426 into master will decrease coverage by 1.01%.
The diff coverage is 36.17%.

@@            Coverage Diff             @@
##           master     #426      +/-   ##
==========================================
- Coverage   94.93%   93.91%   -1.02%     
==========================================
  Files          10       10              
  Lines        2387     2433      +46     
==========================================
+ Hits         2266     2285      +19     
- Misses         80      107      +27     
  Partials       41       41              

Impacted Files	Coverage Δ
toml.go	76.27% <6.25%> (-7.38%)	⬇️
marshal.go	96.28% <100.00%> (+0.07%)	⬆️

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[pelletier]

Thanks for the patch! Impressive we had the getters for so long without the setter 😅

[sclevine]

Thanks for the quick merge! 🎉