Avoid array allocations for generated coercer code

Before this commit the coercer generated following hashes:

    Hash[*[:id, 1, :field, "string"]]

After this commit the coercer generates code with the same semantics:

    Hash[:id, 1, :field, "string"]

This later code is not only shorter but also faster because it allocates
less array objects (no splat).

A small benchmark tries to proof it:

    require 'benchmark/ips'

    Benchmark.ips do |x|
      x.report "hash simple" do
        Hash[:foo, 1, :bar, 2]
      end

      x.report "hash splat" do
        Hash[*[:foo, 1, :bar, 2]]
      end
    end

    =begin

    Calculating -------------------------------------
             hash simple     91144 i/100ms
              hash splat     80099 i/100ms
    -------------------------------------------------
             hash simple  2241513.4 (±1.3%) i/s -   11210712 in   5.002338s
              hash splat  1840887.2 (±1.7%) i/s -    9211385 in   5.005211s
    =end
Merge pull request #51 from splattael/speedup-coercer

Avoid array allocations for generated coercer code