refactor U0 and U1 interfaces

[fbstj]

into putc and getc? with library functions to do proper read/write things and blocking/nonblocking alternatives

[fbstj]

• putc can write a character out directly or buffer/queue it for an interrupt (potentially blocking until it is sent)
• getc can wait for a character from the device (or quickly fail if non are available), wait for an interrupt, or read a buffered/queued character
• puts can putc each character, or queue the string and let the interrupt