Loading dependences on the fly

[nikswamy]

This PR provides --ext fly_deps, an option to scan and load dependences of a module incrementally, just before a single ast declaration is type checked, rather than scanning and loading a module's dependences up front, before the module is checked.

This is especially motivated by wanting to fix a wart in the interactive mode, where adding a dependence while working on a module required restarting the IDE.

A change in behavior with fly_deps is that modules come in scope only at their first reference. So, for example,

module A
let f = e
let g = B.h

the typeclass instances, SMT patterns etc. exported by B are not in scope for checking let f = e.

Another is that friend declarations must be the first declared dependence on a module. Otherwise, we report errors like this:

* Error 308 at /home/nswamy/workspace/everest/pulse/lib/core/PulseCore.Action.fsti(51,0-51,60):
  - Friend dependences must be declared as the first dependence on a module.
  - A non-friend dependence was already found on module
    PulseCore.InstantiatedSemantics.

--ext fly_deps is enabled throughout the F* repo, including in .fst.json config files for VS code. I expect it will become the default behavior at some point, but having it under a flag is to limit regressions in other repositories, until we all get a chance to use this feature and shake out any remaining issues.

[nikswamy]

FYI, fly_deps also works in Karamel and in pulse (with some small changes to ordering for friend declarations)