[#316] Use `IsExpr` only on the front-facing Indigo functions

Description

Problem: using IsExpr in data constructors and for backend functions arguments has no real advantage and has some problems. Mainly, it causes the manipulation of statements and expressions to be require many unwrapping.

Solution: use Expr everywhere except on the functions that the user ultimately interfaces with, making the toExpr conversion at the very beginning. Additionally, this removed the AreExpr, for lack of usage.

Related issue(s)

Resolves #316 (closed)

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