Add BufferField support for Increment/Decrement.

[dewyatt]

Feedback welcome, probably room for improvement here (new to the codebase).

The PCI bus _CRS on my machine requires this in a couple places like:

                CreateDWordField (RES0, 0xB4, GSLN)
                GSMN = ^VLVC.BDSM /* \_SB_.PCI0.VLVC.BDSM */
                GSMX = (^VLVC.TOLD & 0xFFFFF000)
                GSMX--
                GSLN = ((GSMX - GSMN) + One)

    0F63: 8A 52 45 53 30 0A B4 47 53 4C 4E 70 5E 2E 56 4C  // .RES0..GSLNp^.VL
    0F73: 56 43 42 44 53 4D 47 53 4D 4E 7B 5E 2E 56 4C 56  // VCBDSMGSMN{^.VLV
    0F83: 43 54 4F 4C 44 0C 00 F0 FF FF 47 53 4D 58 76 47  // CTOLD.....GSMXvG
    0F93: 53 4D 58 72 74 47 53 4D 58 47 53 4D 4E 00 01 47  // SMXrtGSMXGSMN..G
    0FA3: 53 4C 4E                                         // SLN

(full tables here)

[martin-hughes]

Looks all good so far. Would you mind adding a test case for it? I don't think you could just write a ASL file for it, because aml-tester returns all reads as zero. Instead I think you probably need to copy one of the integration-style tests. The easiest examples to base it on are probably in tests/normal_fields.rs.

Take a look at the "standard handler" in that file - you'll need to provide values for the reads and checked values for the writes, but the test harness will assert that everything is correct.