Added support for search/removal using Expression instances

[webmozart]

This PR makes search/removal in Discovery objects a bit more flexible, which is needed in practice where you may want to search bindings by query/class name.

[webmozart]

ping @stof @tgalopin

[webmozart]

Some more information: With this PR, things like the following are possible:

$bindings = $discovery->findBindings(
    Asset::class, 
    Expr::method('getParameterValue', 'server', Expr::same('localhost'))
);

An alternative solution is to accept a closure instead of Expression instances:

$bindings = $discovery->findBindings(
    Asset::class, 
    function (Binding $binding) { return 'localhost' === $binding->getParameterValue('server'); }
);

However, that solution forces all implementations of Discovery to load bindings into memory whenever a closure is passed. Expression instances OTOH allow, at least in theory, to be transformed into criteria (e.g. DB queries) for the backend of the implementation.

What do you think?

[stof]

I agree that the expression is a better idea than the closure here, given it is for a repository class.

[stof]

I haven't reviewed the implementation though