Allow arbitrary Horn clauses for inter-feature dependencies.

Sometimes it makes sense to enable feature Z if both features Y and feature Z are enabled. Simple example: #6658

[dependencies]
krate = { optional = true, version = "1.2.3" }

[features]
bar = []
foo = [ "krate" ]
"foo+bar" = [ "krate/bar" ]

Another example pops up in native library bindings:

name = "libB-sys"

[dependencies]
libA-sys = { optional = true, version = "4.5.6" } # libA is used by libB to do featX

[features]
vendored = [] # Build the bundled copy of the library instead of using the system one
featX = []
"vendored+featX" = "libA-sys"

It may also make sense to have clauses where resolution fails:

[features]
"do-it-this-way+do-it-that-way" = false

Note that this does not permit requirements of the form "featX or featY must be enabled". The naive algorithm that only enables a feature when all prerequisites are enabled still works (https://www.doc.ic.ac.uk/~rak/papers/kowalski-van_emden.pdf, the "model intersection property")

No implementation files, tests, or entry points are named. Start by tracing Cargo's feature and dependency-resolution logic, then define how conjunction clauses and clauses that fail resolution should be represented and evaluated; done means the TOML examples resolve as described without adding disjunctive requirements.