This is useful for generating FIR filters using code snippets generated at
https://fiiir.com/ (at least those with a rectangular window type; other
window types need additional functions but we can revisit it later if needed)

I think this will come in handy for folks who are using the advanced
features of our audio synthesizer module, synthio.

e.g., the following block now gives highly similar results on ulab
or numpy:

```py
try:
    import numpy as np
except:
    from ulab import numpy as np

# Example code, computes the coefficients of a low-pass windowed-sinc filter.

# Configuration.
fS = 48000  # Sampling rate.
fL = 4000  # Cutoff frequency.
N = 23  # Filter length, must be odd.

# Compute sinc filter.
h = np.sinc(2 * fL / fS * (np.arange(N) - (N - 1) / 2))

# Normalize to get unity gain.
h /= np.sum(h)

# Applying the filter to a signal s can be as simple as writing
# s = np.convolve(s, h)