ext_code and ext_str are inconsistent with ext_code_files and ext_str_files

`ext_code` and `ext_str` take a map of variable names and values, but `ext_code_files` and `ext_str_files` use parallel arrays. In my opinion, the map is a much better experience since it is possible for the user to accidentally transpose values in a larger list without noticing.

Any reason to keep the parallel arrays or would it be ok to switch everything to a map?

I was getting started on a PR to #98 but this style issue needs to be resolved prior to introducing
more inconsistency.

No files or tests are named. Start by comparing the ext_code/ext_str interfaces with ext_code_files/ext_str_files and review the context from issue #98; done means the interface choice is resolved and the related API behavior is made consistent.

Written by the indexing model from the issue text.