serde_userdata: Remove map_err to reduce compile time impact

[s-nie]

map_err has a disproportionate impact on the LLVM IR generated. In generic functions it's beneficial for compile times to avoid it. This reduces the LLVM lines of a crate that heavily uses mlua from 2420270 to 2302295. It could be reduced by a further 50000 lines by manually inlining serde_userdata, but as that would make the code a bit less readable I'm holding off on that.