RegisterSet ctor which takes register defn JSON

[colbynyce-mips]

We have a script which produces JSON files containing various register definitions in them. We would like to be able to give the JSON filenames to a RegisterSet and all the C++ registers will be created automatically.

[colbynyce-mips]

The proposed JSON format looks something like this (array-of-dicts):

{
    "name": "f10",
    "num": 10,
    "desc": "floating point register 10",
    "size": 8,
    "aliases": [
        "fa0"
    ],
    "fields": {
        "sp": {
            "desc": "single precision",
            "low_bit": 0,
            "high_bit": 31,
            "readonly": false
        },
        "dp": {
            "desc": "double precision",
            "low_bit": 0,
            "high_bit": 63,
            "readonly": false
        }
    }
},

Notes:
- The 'fields' are optional and default to {}
- The 'initial_value' is optional and defaults to 0 (and these values are given in hex when not zero)
- The 'enabled' is optional and defaults to True

See the attached JSON files for rv64.
reg_csr.json
reg_fp.json
reg_int.json
reg_vec.json