Add support to concurrency

[danielsn]

RMC does not currently support verification of concurrent code. RMC may spuriously miss bugs that occur when shared data-structures are modified.

Likelihood:

An issue for any concurrent code customers want to verify.

Mitigation:

• Document the limitation.
• Raise a warning if RMC encounters code which uses concurrency

Path to soundness:

• Either document this as not supported,
• or add support.