Fix sleb128_decode overflow checking

[osa1]

Fixes #2625

Refactors decode functions to allow checking overflows.

New tests added for checking edge cases + overflows.

[dfinity-ci]

This PR does not affect the produced WebAssembly code.

[crusso]

LGTM.

Are the option returning versions used on hot paths? Would it be worth writing ones that either panic on overflow or return boolean out parameter indicating success instead of allocating an option?

[osa1]

Are the option returning versions used on hot paths? Would it be worth writing ones that either panic on overflow or return boolean out parameter indicating success instead of allocating an option?

I didn't check the generated code, but in CanCan backend benchmark this reduced cycles by 0.00041% (42,055,832 to 42,055,656).