Since introducing the ctx pointer, the handler is now two words, so setting it can
race with the interrupt firing. On race it's possible for the new handler to be
alled with the old ctx pointer or viceversa.
Rather than documenting this, it's better to split the function in two to make it
obvious to the user that it's not atomic. The user can use a critical section, or
disable/enable the interrupt to avoid races if this is a concern.
The `interrupt` package previously tried to be drop-in compatible with the
`interrupt` package from PACs. THis meant that there was both a PAC-style enum
value `UARTE0` and an embassy-style owned `UARTE0Interrupt` type. This made
things VERY confusing.
This drops compatibility with the PAC, improving the names for embassy interrupts.
- It's no longer possible to call run() reentrantly from within a task (soundness issue)
- it's now possible to spawn Send tasks across threads (SendSpawner, #37)
