Ability to use non-fields in display

Things like this:

#[error(display = "invalid rdo_lookahead_frames {} (expected < {})", _0, MAX_RDO_LOOKAHEAD_FRAMES)]

where MAX_RDO_LOOKAHEAD_FRAMES is a constant in the current module, currently complains that it couldn't find field with this name. Expected behavior would be to try find the field, and if that doesn't work try using it as a literal expression.

Also things like this:

#[error(display = "invalid rdo_lookahead_frames {} (expected < {})", _0, i32::max_value())]

Currently this fails with "All variants must have display attribute.", which is extremely counter-intuitive. I'm guessing that's because of this not returning an error properly: https://gitlab.com/torkleyy/err-derive/blob/88ade958b77b1bc8baf330625e36ca58c6e900b9/src/lib.rs#L215