Consider adding `set` accessor for every `_modify` accessor

Swift will then use the `set` accessor for direct assignments and only use the `_modify` accessor for other mutations, which could be a performance win.

c.f. saeta/penguin#117

Start by locating every `_modify` accessor in the SwiftFusion repository and reading how each accessor is used for direct assignments versus other mutations. Define the complete scope, then verify that each targeted accessor has the intended direct-assignment behavior and that existing mutation behavior remains unchanged.

Written by the indexing model from the issue text.