Add option to deduplicate impl blocks

[nasso]

Same idea as the existing merge_extern_blocks, but for impl blocks.

This is particularly useful when dealing with large "newtype" enums:

Without --merge-impl-blocks:

impl AVCodecID {
    pub const AV_CODEC_ID_NONE: AVCodecID = AVCodecID(0);
}
impl AVCodecID {
    pub const AV_CODEC_ID_MPEG1VIDEO: AVCodecID = AVCodecID(1);
}
impl AVCodecID {
    #[doc = "< preferred ID for MPEG-1/2 video decoding"]
    pub const AV_CODEC_ID_MPEG2VIDEO: AVCodecID = AVCodecID(2);
}
impl AVCodecID {
    pub const AV_CODEC_ID_H261: AVCodecID = AVCodecID(3);
}
impl AVCodecID {
    pub const AV_CODEC_ID_H263: AVCodecID = AVCodecID(4);
}
// ...

With --merge-impl-blocks:

impl AVCodecID {
    pub const AV_CODEC_ID_NONE: AVCodecID = AVCodecID(0);
    pub const AV_CODEC_ID_MPEG1VIDEO: AVCodecID = AVCodecID(1);
    #[doc = "< preferred ID for MPEG-1/2 video decoding"]
    pub const AV_CODEC_ID_MPEG2VIDEO: AVCodecID = AVCodecID(2);
    pub const AV_CODEC_ID_H261: AVCodecID = AVCodecID(3);
    pub const AV_CODEC_ID_H263: AVCodecID = AVCodecID(4);
    pub const AV_CODEC_ID_RV10: AVCodecID = AVCodecID(5);
    pub const AV_CODEC_ID_RV20: AVCodecID = AVCodecID(6);
    pub const AV_CODEC_ID_MJPEG: AVCodecID = AVCodecID(7);
    pub const AV_CODEC_ID_MJPEGB: AVCodecID = AVCodecID(8);
    pub const AV_CODEC_ID_LJPEG: AVCodecID = AVCodecID(9);
    pub const AV_CODEC_ID_SP5X: AVCodecID = AVCodecID(10);
    pub const AV_CODEC_ID_JPEGLS: AVCodecID = AVCodecID(11);
    pub const AV_CODEC_ID_MPEG4: AVCodecID = AVCodecID(12);
    pub const AV_CODEC_ID_RAWVIDEO: AVCodecID = AVCodecID(13);
    // ...
}

[oberrich]

This needs to respect attributes in order to work correctly, I'm working on a larger patch for this: #3088 (comment)
Namely, merge_cfg_attr should be able to take care of this.
We need to make sure all of the passes work on top of each other and we can layer them appropriately.