Allow merging of data structures

[awvwgk]

Merge policy:

• copy key-value pair in case it is not present in table
• drop key-value pair in case it is present in table
• copy subtable in case it is not present in table
• merge subtable in case it is present in table, using same merge policy
• copy array in case it is not present in table
• append array in case it is present in table

[codecov]

Codecov Report

Merging #17 into master will increase coverage by 2.63%.
The diff coverage is 93.42%.

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
+ Coverage   82.05%   84.68%   +2.63%     
==========================================
  Files          35       39       +4     
  Lines        2285     2841     +556     
==========================================
+ Hits         1875     2406     +531     
- Misses        410      435      +25     

Impacted Files	Coverage Δ
src/tomlf/build/merge.f90	85.29% <85.29%> (ø)
test/tftest/build.f90	100.00% <100.00%> (ø)
src/tomlf/type/array.f90	88.00% <0.00%> (-4.31%)	⬇️
src/tomlf/structure/vector.f90	92.14% <0.00%> (-1.20%)	⬇️
test/json2toml.f90	44.82% <0.00%> (ø)
test/json_de.f90	79.62% <0.00%> (ø)
test/json_ser.f90	94.73% <0.00%> (ø)
src/tomlf/utils/convert.f90	93.79% <0.00%> (+0.72%)	⬆️
... and 4 more

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