Fixes #698 #696

This patch finally reimplements partial's block context correctly. The key is to remove all previous blocks and use a separated one for partial.

It corrects behavior of partial parameter, hash and merge_json:

• if parameters is provided, either as path or literal, use it as base value; otherwise use this as base_value
• if hash is provided, always merge it with base value:
  • if base value is associative type: object, array, string, merge them as map
  • if base value is non-associative type, return hash
    otherwise, use base value directly

When partial is rendered, reset blocks.

Tests added.