Support native nodes (metro, time, fft, convolve)

[tamlyn]

These nodes require explicit support from the renderer:

• https://www.elementary.audio/docs/reference/time
• https://www.elementary.audio/docs/reference/metro
• https://www.elementary.audio/docs/reference/fft
• https://www.elementary.audio/docs/reference/convolve

Broken out into smaller tracking issues:

• Support native time node #17
• Support native metro node #18
• Support native fft node #19
• Support native convolve node #20

[txbrown]

@tamlyn I will look into this when I get some time. Likely make a PR for each. Should we break down into separate issues or just cross one by one ?

[tamlyn]

Cool! Make separate issues if you like. It'll feel like more progress to close each one in turn 😄

[txbrown]

Broke this down into per-node issues so each can be implemented/reviewed/closed independently: #17, #18, #19, #20.