rp2: Make rp2_state_machine_exec accept integers.

Currently rp2.StateMachine.exec(instr_in) requires that the instr_in
parameter be a string representing the PIO assembly language instruction
to be encoded by rp2.asm_pio_encode(). This commit allows the parameter
to also be of integral type. This is useful if the exec() method is
being called often where the use of pre-encoded machine code is
desireable.

This commit still supports calls like:
    sm.exec("set(0, 1)")

It also now supports calls like:
    # Performed once earlier, maybe in __init__()
    assembled_instr = rp2.asm_pio_encode("out(y, 8)", 0)
    # Performed multiple times later as the PIO state machine is
    # configured for its next run.
    sm.exec(assembled_instr)

The existing examples/rp2/pio_exec.py and examples/rp2/pio_pwm.py that
exercise the rp2.StateMachine.exec() method still work with this change.

Signed-off-by: Adam Green <adamgrym@yahoo.com>

docs/library/rp2.StateMachine.rst

@@ -82,11 +82,18 @@ Methods
 
 .. method:: StateMachine.exec(instr)
 
-    Execute a single PIO instruction. Uses `asm_pio_encode` to encode the
-    instruction from the given string *instr*.
+    Execute a single PIO instruction.
+
+    If *instr* is a string then uses `asm_pio_encode` to encode the instruction
+    from the given string.
 
     >>> sm.exec("set(0, 1)")
 
+    If *instr* is an integer then it is treated as an already encoded PIO
+    machine code instruction to be executed.
+
+    >>> sm.exec(rp2.asm_pio_encode("out(y, 8)", 0))
+
 .. method:: StateMachine.get(buf=None, shift=0)
 
     Pull a word from the state machine's RX FIFO.