Use David M. Gay's STRTOD instead of system specific atof().

Improves accuracy and compatibility of LOAD DECIMAL! across platforms.
Improves accuracy and compatibility of MONEY! to DECIMAL! conversions across platforms.
Facilitates interpreter porting.
Battle tested e.g. in Linux, Android, Java, Javascript, Python, Firefox, Chrome.
Tested in Windows (0.3.1) with the following log diff:

    [equal? to binary! 2.2250738585072007e-308 #{000FFFFFFFFFFFFF}] progression, succeeded
    [equal? to binary! 2.2250738585072012e-308 #{0010000000000000}] progression, succeeded

    Summary:

    new-successes: 0
    new-failures: 0
    new-crashes: 0
    progressions: 2
    regressions: 0
    removed: 0
    unchanged: 4705
    total: 4707