Final C-API improvements

This includes some minor API changes:

- Remove `sass_data_context_set_source_string`  
  Must be passed to `sass_make_data_context` anyway!
  Leaked memory and file context has no such function!
- Added some functions to take over memory from context:
  - sass_context_take_error_json
  - sass_context_take_error_message
  - sass_context_take_error_file
  - sass_context_take_output_string
  - sass_context_take_source_map_string
- Fixes/changes API for `sass_function_[gs]et_list_entry`
  Should accept value returned by `sass_make_function_list`!
  Switched index number to always be passed as the second argument!
- Fixes a bug when custom function returned the same value