Arithmetic overflows in cosmwasm-std

Some mathematical operations in cosmwasm-std use wrapping math instead of
panicking on overflow for very big numbers. This can lead to wrong calculations in contracts
that use these operations.

Affected functions:

• Uint{256,512}::pow / Int{256,512}::pow
• Int{256,512}::neg

Affected if overflow-checks = true is not set:

• Uint{64,128}::pow / Int{64,128}::pow
• Int{64,128}::neg