Fine-grained delegation of reads and writes in codegen

We currently emit code to read or write a variable in many different places, across many different codegens.

In order to promote reuse of code across codegens, I propose that every time a variable is read or written, this is consistently delegated to the currently active codegen (e.g., `codegen.generate_read` and `codegen.generate_write`). This way, it is less likely that a larger segment of the codegen (e.g., `generate_Tasklet`) needs to be specialized for a specific target, because how variables are read and written is often the only (or one of the only) differences between them.