Arrange for compiler to send to the code generator the complete contents of all files listed on the command line, all first-level dependencies of those files, and parent scopes of those dependencies, but nothing more.  This means it's sometimes possible to compile a schema file without parsing all transitive dependencies.  Also, in 'decode' mode, don't compile annotations, so that the files defining those annotations need not be opened at all.

Fix compiler bugs.  Now test.capnp compiles to an identical schema to the old Haskell compiler, minus intentional changes.

Add utility code for really nice command-line option parsing and start using it in the compiler (still WIP).