Add initial support for converting Quil programs into Cirq circuits

[karalekas]

This PR adds Quil import support in a way analogous to the existing QASM import functionality, by leveraging the Quil parser that already exists in pyQuil.

By using circuit_from_quil from the cirq.contrib.quil_import module, all Quil standard gates, measurements, and simple (non-parameterized) DEFGATEs can be converted to Cirq operations (see below).

Additionally adds Quil output for special cases of TwoQubitDiagonalGate, and fixes a bug in the way DEFGATE blocks were being generated by the Quil outputter.

In [1]: from cirq.contrib.quil_import import circuit_from_quil

In [2]: circuit_from_quil("""
   ...: DECLARE ro BIT[3]
   ...: I 0
   ...: I 1
   ...: I 2
   ...: X 0
   ...: Y 1
   ...: Z 2
   ...: H 0
   ...: S 1
   ...: T 2
   ...: PHASE(pi/8) 0
   ...: PHASE(pi/8) 1
   ...: PHASE(pi/8) 2
   ...: RX(pi/2) 0
   ...: RY(pi/2) 1
   ...: RZ(pi/2) 2
   ...: CZ 0 1
   ...: CNOT 1 2
   ...: CPHASE(pi/2) 0 1
   ...: SWAP 1 2
   ...: ISWAP 0 1
   ...: XY(pi/2) 1 2
   ...: CCNOT 0 1 2
   ...: CSWAP 0 1 2
   ...: MEASURE 0 ro[0]
   ...: MEASURE 1 ro[1]
   ...: MEASURE 2 ro[2]
   ...: """)
Out[2]:
0: ───I───X───H───Z^(1/8)───Rx(0.5π)───@───────@───────────iSwap───────────────@───@───M('ro[0]')───
                                       │       │           │                   │   │
1: ───I───Y───S───Z^(1/8)───Ry(0.5π)───@───@───@^0.5───×───iSwap───iSwap───────@───×───M('ro[1]')───
                                           │           │           │           │   │
2: ───I───Z───T───Z^(1/8)───Rz(0.5π)───────X───────────×───────────iSwap^0.5───X───×───M('ro[2]')───

In [3]: circuit_from_quil("""
   ...: DEFGATE MYZ:
   ...:     1,0
   ...:     0,-1
   ...:
   ...: MYZ 0
   ...: """)
Out[3]:
      ┌     ┐
0: ───│ 1  0│───
      │ 0 -1│
      └     ┘

Related to #1742, #2386, #2983

[vtomole]

LGTM.

[vtomole]

@karalekas Please add some docs to https://github.com/quantumlib/Cirq/blob/master/docs/interop.ipynb