More efficient implementation of "fixed value -> fixed value" mappings

[evansd]

The use case here is using a categorised codelist (e.g. one of the ethnicity codelists) to map codes to categories.

Currently these are implemented using CASE expressions e.g.

CASE
  WHEN code = "abc" THEN "category_1"
  WHEN code = "def" THEN "category_2"
  ELSE "category_3"
END

This appears to have a limit of about 3,000 codes on the TPP database. So far we have not hit this limit.

The way we'll probably want to implement this is to write the mapping to a temporary table and use a JOIN.

This covers the parts of #512 which were not implemented in #1710.