This represents the compilation environment, defined as AST meta_items, Used
for driving conditional compilation and will eventually replace the
environment used by the parser for the current conditional compilation scheme.
Issue #489
Implement "claim" (issue #14), which is a version of "check" that
doesn't really do the check at runtime. It's an unsafe feature.
The new flag --check-claims turns claims into checks automatically --
but it's off by default, so by default, the assertion in a claim
doesn't execute at runtime.
