Bounds handling in rainbow() has unexpected behavior

[totally-not-frito-lays]

Describe the bug
Something interesting I noted in #24 is that when giving invalid input (ie. negative numbers) such as rainbow(-5) the behavior here is different than from CC-Library-Lantern. In CC-Library-Lantern the behavior was rapidly changing colors. In CC-Lantern (this repo), the behavior runs slowly, starting at orange.

This may be a quirk of our implementation of the wait() wrapper function. I'll open up a ticket for this.

To Reproduce
Steps to reproduce the behavior:

1. Open up act3-functions.ino in Arduino IDE
2. Change rainbow(20) -> rainbow(-5)
3. Compile and run the file
4. See slow behavior

Expected behavior
The expected behavior for negative values in the wait() call is to call delay() for 0.25 seconds. This means the colors should cycle through the entire spectrum in 0.25 seconds.

Screenshots
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Desktop (please complete the following information):

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• Browser [e.g. chrome, safari] N/A
• Version [e.g. 22] N/A

Smartphone (please complete the following information):

• Device: [e.g. iPhone6] N/A
• OS: [e.g. iOS8.1] N/A
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• Version [e.g. 22] N/A

Additional context
The easiest fix is to have bounds handling inside the function itself.