Add explicit conversions from bytes32 to uint256 to allow binary operations

[ekpyron]

In solidity 0.5.0 (see argotorg/solidity#4696) we will restrict implicit (and explicit) conversions between number literals and bytesXX types. Among others this will prevent binary operations between bytesXX and hex literals that have a different size (i.e. number of digits) - explicit casts become necessary. This PR adds the necessary explicit conversions to strings.sol.