Rename whatwg::get_encoding to whatwg::encoding_from_label
Rename whatwg::get_encoding to whatwg::encoding_from_label

This indicates that string labels, as opposed to just Encoding objects,
are involved.

nox/rust-encoding,jedisct1/rust-encoding,nox/rust-encoding,servo/rust-encoding,jedisct1/rust-encoding,lifthrasiir/rust-encoding,nox/rust-encoding,lifthrasiir/rust-encoding,tempbottle/rust-encoding,servo/rust-encoding,tempbottle/rust-encoding,lifthrasiir/rust-encoding,tempbottle/rust-encoding