Fix memory leak for filters with keyword arguments

[dylanahsmith]

Alternative to https://github.com/Shopify/liquid-c/pull/157 (see that PR for a description of the problem) Closes #157

This differs from that PR in that it continues to eagerly free the memory to reduce garbage collection.

I tested with the script from https://github.com/Shopify/liquid-c/pull/157 and it actually resulted in a lower RSS:

• 3 runs with this branch:
  • 124832
  • 124724
  • 124924
• on the https://github.com/Shopify/liquid-c/pull/157 branch
  • 130264
  • 131428
  • 130488
• on the master branch, for reference
  • 204172

[dylanahsmith]

Closing in favour of https://github.com/Shopify/liquid-c/pull/157, see the discussion there