extended semantics for defer_signal

[jhellerstein]

Based on #966

Two independent considerations here:

• Do we empty the buffer on new input?
• Do we empty the buffer on new signal?

The current implementation is {input: no, signal: yes}: in a tick when data arrives on the [input] port, append the new data to the contents of the buffer. On [signal], stream out a copy of whatever data is in the buffer and empty the buffer.

Three more possibilities:

{input: yes, signal: no}: in a tick when data arrives on the [input] port, replace the contents of the buffer with the new data from [input] in this tick. On [signal], stream out a copy of whatever data is in the buffer.

{input: no, signal: no}: in a tick when data arrives on the [input] port, append the contents of the buffer with the new data from [input] in this tick. On [signal], stream out whatever data is in the buffer. This is equivalent to cross_join::<'static, 'tick>.

{input: yes, signal: yes}: only produce output if the [input] and [signal] ports both have data in the current tick, producing only the [input] that arrived in this tick. This is equivalent to cross_join::<'tick, 'tick>.

Note that the first two choices replace the buffer in ways that are not equivalent to a join.

[MingweiSamuel]

Related #1373