This commit brings a bunch of optimizations to prettier_print, mostly
in terms of cutting down on allocations.

Most of the common patterns are now using pre-allocated doc nodes
instead of building new ones every time. For nodes that are markers
like BreakParent or Trim, there are now instances that correspond to
them like BREAK_PARENT and TRIM which are used.

For breakables, we now have methods that encapsulate the most common
use cases. These are:

* breakable_space = breakable
* breakable_empty = breakable("")
* breakable_force = breakable("", force: true)
* breakable_return = breakable(" ", indent: false, force: true)

In terms of a couple of breaking changes:

* Trim now strips its whitespace using rstrip! instead of a custom
  gsub!. This means that other forms of whitespace beyond tabs and
  spaces are included. This shouldn't really impact anyone unless
  they're using vertical tab or something in combination with trim
  nodes and wanted them to stay in.
* There is no longer a PrettierPrint::DefaultBuffer class. Since
  there were only ever two implementations, those implementations
  now no longer share a parent.
* PrettierPrint::IndentLevel is now entirely gone. This was mostly
  an implementation detail, and no one should have been relying on
  it anyway. However, it means that the ability to use nest with a
  string literal is now gone as well. It can be created again by
  using seplist though, so the functionality just isn't there in
  the shortcut version. This means we're able to keep track of
  indentation as a single integer again, which drastically
  simplifies the code.
* Strings can now be added directly to the output buffer, which
  means they don't have to be placed into the Text node. This cuts
  down on quite a bit of allocations.

The last optimization is that if_break and if_flat now check if
the parent group is already broken. If it is, then they don't
bother inserting the content into the final tree. This can cut down
on the number of nodes in the final tree by quite a lot.