More general actor address/code validation mechanism

[anorth]

The runtime currently has a few methods for validating caller address and/or code type. The runtime requires that at least one method is called. The interface would be better with a single validation method and an abstract pattern type for declaring the desired pattern.

This would also allow use of the runtime-provided matching mechanism for uses beyond caller validation, such as validating explicitly-provided addresses.

cc @icorderi who has some code for this lying around.

[anorth]

#94 suggests a declarative mechanism rather than the current procedural one.