Use NumericType classes, additional tests, transition to simplejson.

[cgyurgyik]

Closes #464. As discussed in there, The json module automatically converts decimal values to float. simplejson allows for the numbers to be casted directly to Decimal.

Next, this PR adds a NumericType interface, which provide FixedPoint and Bitnum classes. Given a value (in decimal, binary string, or hexadecimal string form), width, and signed-ness, these classes will provide simple getter methods to view different interpretations of the variable, e.g. hex_string(), unsigned_integer(), and so on. Some simple checks are also implemented, such as overflow.

FixedPoint(
   value="1.5",
   width=3,
   int_width=1, 
   is_signed=True
).pretty_print()

will output:

        Signed Fixed Point: 1.5
        ------------------
        Width: 3, IntWidth: 1, FracWidth: 2
        Decimal Class: 1.5
        Fraction Class: 3/2
        Bit String: 0b110
        Hex String: 0x6
        Unsigned Integer: 6

[rachitnigam]

Minor comments. Merge when addressed. I assume you've already tested the changes with calyx-eval?

[cgyurgyik]

Minor comments. Merge when addressed. I assume you've already tested the changes with calyx-eval?

Done.