Support return statements in Laurel functions

[keyboardDrummer]

Changes

Functional:

• Adds a new EliminateReturnsInExpression pass that tries to eliminate return statements.
• Adds not supported message for assert, assume, and local variable declarations without an initializer, that occur in expressions.

Refactoring:

• Introduces a MetaData type abbreviation and refactors the disallowed helper signature
• Renames boogie* variables to core* in translateStmt

Performance:

• Partitions liftImperativeExpressions to skip functional procedures. This does not change the outcome of the pipeline but it might reduce work a bit.

Testing

• Added cases to T3_ControlFlow
• Added T3_ControlFlowError to clarify that assert and assume are not yet supported in functions

[joehendrix]

I left one performance comment, but I don't consider it blocking.

[shigoel]

LGTM in general; left two minor comments aren't blocking.