rpc: Use the real generational-arena crate

Now that they've fixed their description of the MPL-2.0 license we can take this dependency with more comfort.

This branch also changes the status of MPL-2.0 in our check_license code from "always allow" to "allow when explicitly permitted". Unfortunately, this means we need to list option-ext for now, since the directories crate depends on dirs-sys, which depends on option-ext.