Fix handling of signed bit fields

[AArnott]

Closes #987

Sample code gen for an int bitfield that demonstrates both multi-bit and single-bit values:

internal partial struct FLICK_DATA
{
    internal int _bitfield;

    /// <summary>Gets or sets bits 0-4 in the <see cref="_bitfield" /> field. Allowed values are [-16..15].</summary>
    internal sbyte iFlickActionCommandCode
    {
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        readonly get => (sbyte)((this._bitfield << 27) >> 27);
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        set
        {
            Debug.Assert(value is >= (sbyte)-16L and <= (sbyte)15L);
            this._bitfield = (int)((this._bitfield & unchecked((int)~0x0000001F)) | ((int)(value & 0x0000001F) << 0));
        }
    }

    /// <summary>Gets or sets bits 5-7 in the <see cref="_bitfield" /> field. Allowed values are [-4..3].</summary>
    internal sbyte iFlickDirection
    {
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        readonly get => (sbyte)((this._bitfield << 24) >> 29);
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        set
        {
            Debug.Assert(value is >= (sbyte)-4L and <= (sbyte)3L);
            this._bitfield = (int)((this._bitfield & unchecked((int)~0x000000E0)) | ((int)(value & 0x00000007) << 5));
        }
    }

    /// <summary>Gets or sets bit 8 in the <see cref="_bitfield" /> field.</summary>
    internal bool fControlModifier
    {
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        readonly get => (this._bitfield & 0x00000100) != 0;
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        set => this._bitfield = (int)(value ? this._bitfield | 0x00000100 : (this._bitfield & unchecked((int)~0x00000100)));
    }
}

And here's an unsigned byte bitfield divided up:

internal partial struct _BM
{
    internal byte _bitfield;

    /// <summary>Gets or sets bits 0-1 in the <see cref="_bitfield" /> field. Allowed values are [0..3].</summary>
    internal byte Recipient
    {
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        readonly get => (byte)((this._bitfield >> 0) & 0x03);
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        set
        {
            Debug.Assert(value is <= (byte)3L);
            this._bitfield = (byte)((this._bitfield & unchecked((byte)~0x03)) | ((byte)(value & 0x03) << 0));
        }
    }

    /// <summary>Gets or sets bits 2-4 in the <see cref="_bitfield" /> field. Allowed values are [0..7].</summary>
    internal byte Reserved
    {
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        readonly get => (byte)((this._bitfield >> 2) & 0x07);
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        set
        {
            Debug.Assert(value is <= (byte)7L);
            this._bitfield = (byte)((this._bitfield & unchecked((byte)~0x1C)) | ((byte)(value & 0x07) << 2));
        }
    }
}

Whether signed or unsigned, single-bit fields are expressed as a bool property.

[AArnott]

@tannergooding @riverar I'd very much appreciate your reviews.
I suggest focusing on the tests I added, as those are the most readable. If we have adequate coverage, the code gen should be good.
I can also paste in actual code gen snippets if you specify which field types / lengths you're interested in.

[riverar]

Was on my TOREVIEW list, didn't realize the time fuse was so short. I'll still take a peek as soon as possible.

[AArnott]

Thanks. Yes I'll absolutely respond to any review you offer, @riverar.