Add SafeCast variants for signed integers

[nventuro]

SafeCast currently has support for downcasting uints and casting them to int256, but there's no safe way to downcast signed integers.

This is slightly trickier because the two's complement integer representation has different ranges for positive and negative numbers, but should be easy to do anyway.

[frangio]

You're talking about a function functions like toInt128(int256 x) right?

[nventuro]

Precisely, yes. toInt8(int256 x) for example needs to check that -128 <= x <= 127.

[julianmrodri]

@nventuro @frangio I can tackle this if you are OK. Just let me know.

[julianmrodri]

@nventuro @frangio I gave it a shot. Submitted PR #2243 Here is how one of the functions look for the example you mentioned above. Any chances or adjustments that need to be made please let me know. Thanks!

/**
 * @dev Returns the downcasted int8 from int256, reverting on
 * overflow (when the input is less than smallest int8 or greater 
 * than largest int8).
 *
 * Counterpart to Solidity's `int8` operator.
 *
 * Requirements:
 *
 * - input must fit into 8 bits.
 */ 
function toInt8(int256 value) internal pure returns (int8) {
    require(value >= -2**7 && value < 2**7, "SafeCast: value doesn\'t fit in 8 bits");
    return int8(value);         
 }