-safe-string compatibility

[rgrinberg]

Fixes lambda-term for 4.06.0

[rgrinberg]

This works but unfortunately break compatibility with the old Camomile (0.8.5) which means that this release won't work on 4.02.3.

This is fixable in 2 ways:

• We can remove the problematic asserts that don't work with the new Camomile
• Backport Camomile to work with 4.02.3

I will look for a solution in a separate PR.

[rgrinberg]

I've restored 4.02.3 compatibility in Camomile. So lambda term will require Camomile 0.8.7 on 4.02.3

[Leonidas-from-XIV]

I tried building it on 4.05 but it is broken:

File "src/lTerm_unix.ml", line 342, characters 32-51:
Error: This expression has type bytes but an expression was expected of type
         string
      ocamlc src/lTerm.{cmo,cmt} (exit 2)

and

File "src/lTerm.ml", line 334, characters 32-51:
Error: This expression has type bytes but an expression was expected of type
         string

Removing safe-string from the src/jbuild fixes it for me, but obviously this is not the proper solution.

Oh, turns out it builds with camomile 0.8.7... @rgrinberg wouldn't it be best to just set 0.8.7 as minimum dependency version here?