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.
