Add dist_density() and dist_kernel_density()

[mitchelloharawild]

Similar to dist_quantile()/dist_percentile(), for density. Also consider dist_cdf() for completeness.

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dist_density(density, at)

dist_kernel_density(density, at, kernel = dist_*(), x): x is optionally the data for some more time efficient methods (less space efficient)

OR

dist_kernel(kernel = dist_*(), x)?