swf: Re-organize Tag type and reader/writer methods

[Herschel]

Doing some spring cleaning of swf. This is mostly busy-work with no functional change.

• Make all variants of Tag follow the pattern of Tag::TagName(TagStruct).
  • Previously most were structs already, but some were inline fields Tag::TagName { foo, bar }.
  • Make this consistent both for aesthetics and for ease of use (a struct per variant allows you to pass it to a function, add methods on the struct itself, etc.).
• Move Tag to tag.rs.
• Re-arrange the Reader/Writer methods.
  • Previously there was a lot of chunky code blocks in the massive match statements.
  • Now every tag variant has a read_foo/write_foo method.
  • Alphabetized Tag + roughly organized methods.
• Remove uses of ref in Writer.

The motivation is I'm working towards finally adding support for streaming all of the tags/bytecode in swf.