fix build issues on different rust versions

[Geal]

• clamp is not available in 1.48
• since Use niche-filling optimization even when multiple variants have data. rust-lang/rust#94075 that shipped in 1.65, a test asserting the size of IResult is failing because the structure is now small by 8 bytes

[Xiretza]

#1549 should take care of the clamp() and another issue in the same area.

[Geal]

right, that works better