[RFC] Compiler warnings DSL

Hi,

The OCaml compilers have several options to control what warnings are active, and which ones are fatal.

However, it's a bit difficult to remember all the details:

• how do -w and -warn-error interact
• what happens when -w or -warn-error is passed several times
• what do numbers mean
• what do letters mean, including uppercase and lowercase

This can be eased by adding a DSL that's available in places where (flags) is accepted:

(warnings
 (enabled ..)
 (disabled ...)
 (fatal ...))

Exact syntax & semantics are to be discussed, but the general idea is to convert this into a flag that will be combined with the rest of the command line.

cc @rgrinberg

Start by reviewing Dune's handling of (flags) and the OCaml compiler options -w and -warn-error; the issue names no files or tests. Done means agreeing on syntax and semantics for enabled, disabled, and fatal warnings and converting the DSL into a combined command-line flag.