Implement $size, which counts elements of an array, in SBE:

• Add stage builder code for translating $size from the classic Expression tree representation to SBE EExpression.
• Add new VM bytecode primitives if necessary. (For this task, likely unnecessary)
• Add unit tests for stage builder implementation
• Remove existing code that prohibits SBE from being used when $size is present
• Review existing integration tests for $size and improve as needed
• Review existing perf tests and improve if needed