[#314] move `#=` to Indigo's frontend and make it a synonym for `//->`

Description

Problem: we had a #= operator that was a synonym of /->, but the former is no longer usable and the latter is not used in favor of //-> (in the frontend).

Solution: move #=to Indigo's frontend and make it a synonym for//->` instead.

Related issue(s)

Resolves #314 (closed)

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