Deprecations and type inference improvements for template resolvers

[gsteel]

The return type for a template resolver should simply be string. Any kind of failure should cause an exception, ideally something like TemplateNotFound.

All existing resolvers are marked as @final and there are several additional deprecations to clean up un-documented behaviour.

The baseline should be relatively even - most of the solved issues have been replaced with deprecation issues + some new issues surfaced by the type inference improvements.

Deprecations

• Additional protected properties relating to already deprecated stream wrappers
• public error constants that will be unnecessary in 3.0 when resolvers throw exceptions instead of returning false.
• (Undocumented) Using TemplateMapResolver::add(string, null) to remove individual templates - this now triggers E_USER_DEPRECATED
• AggregateResolver::getLastSuccessfulResolver() - likely exists only for tests
• AggregateResolver::getLastLookupFailure() - again, probably only there for tests
• TemplatePathStack::getLastLookupFailure() - as above

[gsteel]

@froschdesign Will this be good-to-go once #216 is merged?