add MAP/PLUCK functions

[vlazar]

For PLUCK function I had to directly access hash keys with item[attr.to_sym] || item[attr]. Using Dentaku node.value(context) is not working for symbol keys as described here: #218 (comment)

I'll try to figure out how to fix this issue and use Dentaku in PLUCK function definition to pluck keys form hashes.

[codecov-io]

Codecov Report

Merging #221 into master will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #221      +/-   ##
==========================================
+ Coverage   96.46%   96.54%   +0.08%     
==========================================
  Files          58       60       +2     
  Lines        1641     1680      +39     
==========================================
+ Hits         1583     1622      +39     
  Misses         58       58              

Impacted Files	Coverage Δ
lib/dentaku/ast.rb	100.00% <100.00%> (ø)
lib/dentaku/ast/functions/map.rb	100.00% <100.00%> (ø)
lib/dentaku/ast/functions/pluck.rb	100.00% <100.00%> (ø)

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[vlazar]

I've fixed the issue with symbol keys not woking for PLUCK similar to how it works for MAP. See my last commit. I hope this is acceptable implementation (building expression in function seems weird, but I don't know what would be a better way).

Basically the issue boils down to this:

pp Dentaku::FlatHash.from_hash("u" => { age: 44 }) # => {"u.age"=>44}
pp Dentaku::AST::Identifier.new(Dentaku::Token.new(:identifier, "u.age")).value("u.age" => 44) # => 44

pp Dentaku::FlatHash.from_hash({ age: 44 }) # => {:age=>44}
# pp Dentaku::AST::Identifier.new(Dentaku::Token.new(:identifier, "age")).value({ age: 44 }) # => UnboundVariableError
pp Dentaku::AST::Identifier.new(Dentaku::Token.new(:identifier, :age)).value({ age: 44 }) # 44

pp Dentaku::FlatHash.from_hash(u: { age: 44 }) # => {:"u.age"=>44}
# pp Dentaku::AST::Identifier.new(Dentaku::Token.new(:identifier, "u.age")).value({:"u.age"=>44}) # => UnboundVariableError
pp Dentaku::AST::Identifier.new(Dentaku::Token.new(:identifier, :"u.age")).value({:"u.age"=>44}) # => 44

Since Dentaku::FlatHash.from_hash for PLUCK was originally called on each collection item and not { "i" => item } the keys in hash were not converted to strings. And Dentaku::AST::Identifier#value is just fetching a value from hash. So for symbol keys the value wasn't found by string key in my original implementation of PLUCK which was cargo culted from MAP.