Require a colon before 'group' or 'union' except for unnamed unions as well as the legacy case of a named union with an ordinal (where the colon is optional).

Pull TextBlob out of capnpc-capnp and into KJ as kj::StringTree.  Use it in the stringification code to make that code cleaner and avoid using iostreams.

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.

Revert "Work around weird behavior in GCC 4.8 where it thinks this destructor's declaration and definition hase mismatching noexcept specs even though neither one has a noexcept spec at all."

This reverts commit c088dd95.

Reverted in favor of dwrensha's more complete fix.

Work around weird behavior in GCC 4.8 where it thinks this destructor's declaration and definition hase mismatching noexcept specs even though neither one has a noexcept spec at all.