Add native JVMTI agent with allocation sampling

[gharrma]

For now the agent is used only in tests, not in production.
Later it may be used to estimate allocations for traced methods.

There are some things left to figure out before submitting this:

• Plan how to build and release the native agent for all three platforms.
• Address remaining TODO in agent.cpp
• Decide whether estimating allocations is useful enough to be worth the added complexity