use appropriate loop when assigning header metadata to document
fix rowspan assignment for compatibility with Opal
minor optimization for partintro in docbook backend
fix while loop exit for Opal compatibility
remove sub of Set, define to_set properly
add SecurityError impl for Opal compatibility
remove opal_ext/set.rb, to_set is now implemented in Opal
resolves #579 specialchars subs alias, shorthand subs

- add specialchars as an alias for the subs key specialcharacters
- resolve single-letter subs for inline usages such as pass:[]