ENH: new function `replace`

[lucascolley]

From https://hackmd.io/zn5bvdZTQIeJmb3RW1B-8g?both#Updates-from-the-sparse-array-ecosystem-Willow, @willow-ahrens suggested an xp.replace(array, old, new) function, which could have the agnostic implementation xp.where(xp.equals(...), old, new ).

@willow-ahrens if you'd like to work on this, we have contributor docs at https://data-apis.org/array-api-extra/contributing.html.

[kgryte]

Bikeshed: might be worthwhile considering the current constellation of names among various array libraries and whether we want to introduce new vocabulary or build on existing vocabulary. E.g., NumPy already has fill, so fill_equal could be an option.

Additionally, the proposed operation is a functional equivalent to

x[x==old] = new

which is already supported by the standard vis-a-vis boolean indexing. In principle, one could have a dedicated kernel for this operation, thus avoiding the intermediate boolean array, but there is a question whether this API is actually needed given support for boolean indexing.