Reducing some dependencies from the compiler into ulib

[mtzguido]

From the commit message:

Machine integers in the F* library depend on several modules, such as
Math.Lemmas, BitVector, Calc, etc, which in turn bring in dependencies
to many more.

Since the compiler sources mention these modules, we are bringing in
thees dependencies. The main reason the compiler uses these modules is
to use UInt32, which is the representation for characters in F*.

However, we do not really need any fact about them, we only need to use
the type and some extremely basic operations such as going to and from
int.

So, to fix this, we play a dependency analysis trick by introducing some
rather empty modules in src/basic/boot, which take priority in the
dependency analysis, and effectively hide the ones in ulib/ and their
dependencies. This reduces the dependency graph considerably.

This PR also adds a rule to easily obtain a PDF of the dependency graph of the compiler. Here's before and after (maybe not a "considerable" reduction come to think of it :) )

depgraph_old.pdf
depgraph_new.pdf

This also saves about a minute of (sequential) bootstrapping time.

[nikswamy]

This looks great, Guido ... thanks!

I've kicked an everest build as well (on everest branch nik_wf) just to check that we don't have anyone relying on BaseTypes in HACL etc.

If it all comes back green, please merge

[mtzguido]

I've kicked an everest build as well

oops, good point! I now see everparse uses it. Feel free to kill that build as I make a PR for it.

[mtzguido]

Got a local green, merging.