Purity Enforcement

[dsainati1]

Closes https://github.com/onflow/cadence/issues/1806

Implements the enforcement for the purity analysis implemented in https://github.com/onflow/cadence/pull/1818

Currently, impure operations (those operations banned in pure contexts), are:

• Calling an impure function (including any functions that modify account state or storage or log values)
• Emitting an event
• Writing to any resources
• Writing to any references
• Indexed assignment or writes to any variables not statically knowable to have been defined in the current function's scope

This is very restrictive; however given that the purpose of pure functions is to allow some operations to be performed in function pre and post-conditions, it is unlikely that support for a more complex analysis is worth the complexity it would require. As it stands, this allows calls to pure functions on composites, creation of composites with pure initializers, and any operations that don't perform an assignment.

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