* Mark EntryCustom::new_custom as unsafe
Passing a badly-behaved `load` function can invoke undefined behavior.
* Document required feature for Entry
* Support linking Vulkan directly
This is the preferred pattern in most environments when an application
cannot function without Vulkan, as it saves the libloading dependency,
eliminates an error case, and makes the Vulkan dependency visible to
the OS.
* Rename libloading feature to "loaded"
* Link by default
* Guide users towards linking the loader directly
* Remove unnecessary error type
InstanceError::LoadError was never constructed.
* Unify entry types
Simplifies the interface and allows a bunch of code to become
monomorphic.
* ci: Test docs in addition to `cargo t --all-targets`
Unfortunately docs are not explicitly (build-)tested as part of
`--all-targets` allowing broken code to slip in as shown by #390.
Also remove the `rust` listing type which is the default, leaving only
`no_run` (until the CI has a loadable Vulkan library).
* ash: Fix errors and warnings in (now tested) documentation comments
