serialization: facilitated rvalue serialize() by branched write().

serialization/support: made SerSequence serialize() take && again.
tool_build/package_compiler: added forgotten LookupNode definition.¹

1: This should have been included in commit
2c3a127bdc84bbb64c1dbcf6b9e28add63806c18.
tool_build/resource_compiler: return bool from compile_func.
tool_build/package_compiler: added path(); tidy.
tool_build/package_compiler: create .compiled directory in create_stub().
tool_build/package_compiler: made metadata serialization uniform.

tool_build/package_compiler: push entries into _lookup after reading.
tool_build/package_compiler: added write().