This PR reworks the unix I/O backend, removing runtime reference
counting/borrow checking and optimizing away the hashmap in favor of a
static array, with an unlikely fallback vec.
The only reason the fallback vec is there is because unlike the
`io_uring` module, we cannot simply index into the array with the fd as
the OS could theoretically give us a fd up to I believe 1024 so keeping
an array of that size for a few elements is unnecessary.
Closes#940
