Check unchecked arithmetic for externally provided values

[notmandatory]

"All across the crate, unchecked arithmetic is used. The unchecked arithmetic routines of rust-bitcoin's Amount type will panic if the result of an operation overflows. This is an issue for externally-provided values, such as for instance recipient amounts or fees"

[oleonardolima]

I guess this is partially solved (if not fully -- needs checking) by bitcoindevkit/bdk#1763.