servo: Merge #16728 - style: Rewrite calc to be cleaner and support a…

…rbitrary expressions (from emilio:recalc); r=waffles

This improves Servo's calc support compliant with[1], and makes it cleaner and
more straight-forward. (also fixes #15192)

[1]: w3c/csswg-drafts#1241

Source-Repo: https://github.com/servo/servo
Source-Revision: 7fc01437f4c8935951add61a76230131134382f8

UltraBlame original commit: d5aa4f2a3c9bd947292ce93a82c37c552cfe6b1e