Feature request: let end-users plug in their own backend code generators written in a template language (ST4)

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Problem:
Current implementation of the P compiler relies on backend code generators hand-written in C#, making it hard for end-users to write their own code generators. It also makes current code generators harder to write and maintain.

Solution:
String Template 4 (ST4) offers a simple way of building user-defined code generators. Integrating with ST4 is a straightforward task with very clear "handoff" between the host program and the template engine. The host program needs to read the templates, make some version of a preprocessed program tree available to them, and invoke the templates.

Notes:

• Making this pluggable will likely require adding some sort of a config file that links template names to the names of output files.
• There are multiple ports of ST4 to C#, such as this one.
• My fork of the C# port of ST4 adds back the use of embedded resources as templates, which may be helpful in re-defining some of the existing backend generators in ST4.