Optimize select actions to avoid heap allocations

[straight-shoota]

I'm also surprised to learn that a select expression creates a class for each of the actions (like SendAction) so memory is allocated on every select. Really bad! I think it should be done in a way that no memory is ever allocated. Like, probably SendAction should be a struct, and we create a static array to hold those values in the stack. Then use pointers to refer to those. I think Go might be doing the same thing.

Originally posted by @asterite in #12694 (comment)

Related: #12756