Look into using string builder rather than array concatenation for query building

[fridgerator]

It seems like String builder shoud have better performance?

Example : https://github.com/Crecto/crecto/pull/180/files#diff-0813040cd5dc1286ba01ba171a504bd8R235

[fridgerator]

almost twice as fast with this simple example:

require "benchmark"

Benchmark.bm do |x|
  x.report("arrays : ") do
    100_000.times do
      q = ["WHERE"]
      q.push "user.id=?"
      q.push "and user.name=?"
      q.join(" ")
    end
  end

  x.report("string builder : ") do
    100_000.times do
      String.build do |str|
        str << "WHERE"
        str << " user.id = ?"
        str << " and user.name=?"
      end
    end
  end
end

compiled with --release --no-debug

                        user     system      total        real
arrays :            0.030000   0.040000   0.070000 (  0.044140)
string builder :    0.020000   0.030000   0.050000 (  0.024460)