Some helpers to annotate the AST so merlin can decide which branches to look at and which branches to ignore.

### Annotations merlin understand

`val hide_attribute : Ppxlib__.Import.attribute`

Adding this `[@merlin.hide]` attribute on a piece of AST "hides" it from merlin: it tells merlin not to consider that branch if another piece of AST with the same location exist.

`val focus_attribute : Ppxlib__.Import.attribute`

Adding this `[@merlin.focus]` attribute on a piece of AST tells merlin to prefer it to any other piece of AST when several have the same location.

### Helpers

The following functions add the corresponding attribute (defined above) to specific pieces of AST.

`val hide_pattern : Ppxlib__.Import.pattern ‑> Ppxlib__.Import.pattern`
`val focus_pattern : Ppxlib__.Import.pattern ‑> Ppxlib__.Import.pattern`
`val hide_expression : Ppxlib__.Import.expression ‑> Ppxlib__.Import.expression`
`val focus_expression : Ppxlib__.Import.expression ‑> Ppxlib__.Import.expression`