FSI caching

I propose we cache the compiled scripts for FSI so that when they are run again without any changes, it doesn't have to recompile from scratch.

The existing way of approaching this problem in F# is to use Fake's script runner.

## Pros and Cons

The advantages of making this adjustment to F# are that we can use FSI for scripting.

The disadvantages of making this adjustment to F# are more work.

## Extra information

Estimated cost (XS, S, M, L, XL, XXL): M? S? not sure. Depends on also if we have to cleanroom this or if the Fake team is interesting in collaborating.

Related suggestions: (put links to related suggestions here)

## Affidavit (please submit!)

Please tick this by placing a cross in the box:
* [X] This is not a question (e.g. like one you might ask on [stackoverflow](http://stackoverflow.com)) and I have searched stackoverflow for discussions of this issue
* [X] I have [searched both open and closed suggestions on this site](http://github.com/fsharp/fslang-suggestions/issues) and believe this is not a duplicate
* [X] This is not something which has obviously "already been decided" in previous versions of F#. If you're questioning a fundamental design decision that has obviously already been taken (e.g. "Make F# untyped") then please don't submit it.

Please tick all that apply:
* [X] This is not a breaking change to the F# language design
* [X] I or my company would be willing to help implement and/or test this

## For Readers

If you would like to see this issue implemented, please click the :+1: emoji on this issue. These counts are used to generally order the suggestions by engagement.

Start by investigating FSI's script execution and the existing Fake script runner, which the issue identifies as a related approach. Define how compiled scripts would be reused only when unchanged and how cache invalidation should work. Done means repeated runs of an unchanged script avoid recompilation without changing FSI behavior.

Written by the indexing model from the issue text.