Make it easier to use show/inspect/... from the REPL

[smarter]

Right now it's a bit awkward:

$ mill -i
@ inspect(replApplyHandler.evaluator, "main.compile")()
[1/1] inspect 
main.compile(ScalaModule.scala:125)
Inputs:
[...]

It'd be nicer if we could do:

@ inspect("main.compile")()

Or even:

@ inspect(main.compile)()

(It would also be nice to avoid the extra () but that's a separate issue)

The first should be achievable by having the evaluator implicitly available in scope and changing the signature of show/inspect/... to take the evaluator implicitly. @lihaoyi on Gitter said:

Mill's routing macros do not support multiple arglists IIRC
but they can be made to do so'
the fork i use in Cask supports them

[lihaoyi]

This can probably be done most easily by changing the code wrapper template to wrap the expression in an additional e.g. evalTask(...) call, which would leave T <!< Tasks alone and evaluate T <:< Tasks the same way that () does so

[lihaoyi]

although it might still require patching ammonite's Preprocessor.scala to provide to appropriate hook for us to do this wrapping, since we may need to wrap at an expression level rather than at a block level so e.g. main.compile; main.test properly evaluates both tasks and stores their results in local bindings.

Apart from inspect and show, this would make working with the rest of Mill's tasks easier in the mill -i REPL as well

[lihaoyi]

This is now moot since #2377 removed the build REPL entirely due to lack of usage