add 1-D convolution

[philipp-baumann]

from https://twitter.com/BenOgorek/status/1696855672548167842/photo/1

w <- c(0, 0, 1, 1, 0)
y <- c(1, 2, 3, 4, 5)

convolve(y, rev(w), type = "open")[2:(length(y) + 1)]

[philipp-baumann]

convolve uses fft() internally.

[philipp-baumann]

use future::future_lappy() and batching.