We introduce the `with_lock` combinator that locks a mutex in an atomic fashion.
This ensures that exceptions thrown by signals will not leave the system in a
deadlocked state.

This tends to confuse the OCaml compiler, for good reasons. Indeed, if
there are mutable fields, the generated code cannot wait for the function
to be fully applied. It needs to recover the value of the mutable fields
as early as possible, and thus to create a closure.

Example:

    let foo {bar} x = ...

is compiled as

    let foo y = match y with {bar} -> fun x -> ...

We introduce a new package structure for Coq:

- `coq-core`: Coq's OCaml tools code and plugins
- `coq-stdlib`: Coq's stdlib [.vo files]
- `coq`: meta-package that pulls `coq-{core,stdlib}`

This has several advantages, in particular it allows to install Coq
without the stdlib which is useful in several scenarios, it also open
the door towards a versioning of the stdlib at the package level.

The main user-visible change is that Coq's ML development files now
live in `$lib/coq-core`, for compatibility in the regular build we
install a symlink and support both setups for a while.

Note that plugin developers and even `coq_makefile` should actually
rely on `ocamlfind` to locate Coq's OCaml libs as to be more robust.

There is a transient state where we actually look for both
`$coqlib/plugins` and `$coqlib/../coq-core/plugins` as to support
the non-ocamlfind plus custom variables.

This will be much improved once #13617 is merged (which requires this
PR first), then, we will introduce a `coq.boot` library so finally
`coqdep`, `coqchk`, etc... can share the same path setup code.

IMHO the plan should work fine.

We also spill (some) non-generic arguments and initialization code
out of coqargs and to coqtop, namely colors for the terminal. There are
more of these, left to later commits.

This component holds the code for initializing Coq:
- parsing arguments not specific to the toplevel
- initializing all components from vernac downwards (no stm)

This commit moves stm specific arguments parsing to stm/stmargs.ml