[#237] Include `KnownValue` into `IsExpr`

Description

Problem: There seems to be no case of IsExpr ex a where KnownValue a constitutes a problem, however we are required add the latter every time we use the former (where a is generic).

Solution: Include KnownValue a into IsExpr ex a and remove unnecessary use of KnownValue.

Related issue(s)

Resolves #237 (closed)

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