The existing `per_stage_map` field of MSL backend options specifies
resource binding maps that apply to all entry points of each stage type.
It is useful to have the ability to provide a separate binding index map
for each entry point, especially when the same shader module defines
multiple entry points of the same stage kind.
This patch replaces `per_stage_map` with a new `per_entry_point_map`
option where resources are keyed by the entry-point function name.
We can safely assume that deserialize is enabled for all output tests.
Supporting other modes of operation has low utility.
This change also adds an option to skip the explicit types for WGSL output.
