* string to []rune
* string to []byte
* []rune to string
* []byte to string
* any pointer to uintptr

The string, []rune, []byte conversion pairs aligns this to the go
language.
The pointer -> uintptr conversion pair is symmetric to the uintptr ->
pointer that we already have.

Also lets the user specify any size for byte array types instead of
just the ones already used by the program, this can be used to read
arbitrary memory.

Fixes #548, #867