optimize prop_set by eschewing instance_exec

[froydnj]

Motivation

Turns out this way is ~10% faster (!).

Test plan

See included automated tests.

[froydnj]

We have a policy of testing changes to Sorbet against Stripe's codebase before
merging them. I've kicked off a test run for the current PR. When the build
finishes, I'll share with you whether or how it failed. Thanks!

Stripe employees can see the build results here:

→ https://go/builds/bui_PX5EKuGIyI6ltR
→ https://go/builds/bui_PX5EIsm0YBh4WJ
→ https://go/builds/bui_PX5FXM4wLaF79c

This change also has sorbet-runtime changes. Those tests:

→ https://go/builds/bui_PX5OLYOrnziELw

[jez]

We have a policy of testing changes to Sorbet against Stripe's codebase before
merging them. I've kicked off a test run for the current PR. When the build
finishes, I'll share with you whether or how it failed. Thanks!

Stripe employees can see the build results here:

→ https://go/builds/bui_PX5JGW99mvkX3m
→ https://go/builds/bui_PX5Jh9I9TWDP5k
→ https://go/builds/bui_PX5JfPwwawhUDj

This change also has sorbet-runtime changes. Those tests:

→ https://go/builds/bui_PX5PMYr1nsy0s8