Make line commit unconditional and remove try_commit_line macro

[nicoburns]

This refactors and simplifies the line-breaking code.

Changes made:

• Eliminate the "fake conditionality" from the try_commit_line function (it returned a bool as if it was conditional (which historically it used to be), but in reality it was always returned true). Rename it just commit_line to reflect this reality.
• Take advantage of this non-conditionality to make start_new_line call commit_line internally and remove the try_commit_line macro (the macro was to reduce boilerplate which no longer exists if commit_line is called from start_new_line).
• Breaking (low-level API): Make append_cluster_to_line increment self.cluster_idx (callers should no longer increment this themselves)
• Breaking (low-level API): Make append_inline_box_to_line increment self.item_idx (callers should no longer increment this themselves)