Add test of compiler's error-checking.
Add some missing type checks in dynamic.c++.
Make sure reading far pointers is thread-safe.
Update some todos
Fix problem in evolution-test.
Expand help text re: --packed.
Allow implicit convertion between StringPtr/Text::Reader and std::string.
Delete default construcrtor on builders, so that people don't incorrectly try to just declare a Builder on the stack and then start setting its fields.  Instead, a Builder can be explicitly assigned to nullptr, which makes it clearer that it isn't safe to use.