servo: Merge #4815 - Lowercase DOM getters at compile time, fixes #4728 (from Swatinem:lowercasegetters); r=Manishearth

The implementation was copied directly from
rust-lang/rust#16636
and updated for rust changes, so the credit goes to Manishearth

Source-Repo: https://github.com/servo/servo
Source-Revision: 902c16497c40684930819693a7e90f0862eb7f56

UltraBlame original commit: 1b9c7a69c746676dcf68b519877913b5b2186a56

Author: marco-c