Allow variables in the payload of `%{read*:<path>}`

[MisterDA]

The payload for %{read:<path>} (also read-lines and read-strings) is treated as strings but there is no reason is couldn't be expanded further.

[rgrinberg]

Yeah, this is a good point. Unfortunately, our little DSL for pforms is quite limited. So in order to implement this feature we need an alternative way (preferably using sexps) to express such pforms.

[nojb]

One possibility is to extend the existing syntax to allow %{<sexp>}. Macros %{foo:bar} could be considered as shorthands for %{(foo "bar")}. It is not very pretty though.

[MisterDA]

I suppose doing %(<sexp...>) could break user code without a version gate. That could be just more pleasant than using both curly and curvy parentheses