A fairly regular CopySlice simplification

[msooseth]

Description

This simplifies away some of the ConcreteBuf when we CopySlice from a WriteWord+ConcreteBuf combo. If the ConcreteBuf is larger than what we will copy, the last parts of the ConcreteBuf is useless.

Without:

            (CopySlice
              srcOffset: 0x0
              dstOffset: 0x0
              size:      0x40
              src:
                (WriteWord
                  idx:
                    0x0
                  val:
                    (WAddr
                      (SymAddr "arg1")
                    )
                )
                (ConcreteBuf
                  Length: 96 (0x60) bytes
                  0000:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00   ................
                  0010:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00   ................
                  0020:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00   ................
                  0030:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00   ................
                  0040:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00   ................
                  0050:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 80   ................
                )
            )
            (ConcreteBuf "")

With:

            (CopySlice
              srcOffset: 0x0
              dstOffset: 0x0
              size:      0x40
              src:
                (WriteWord
                  idx:
                    0x0
                  val:
                    (WAddr
                      (SymAddr "arg1")
                    )
                )
                (ConcreteBuf
                  Length: 64 (0x40) bytes
                  0000:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00   ................
                  0010:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00   ................
                  0020:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00   ................
                  0030:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00   ................
                )
            )

Notice that there was no need for all that 96B ConcreteBuf -- the CopySlice will cut it out anyway. So we only keep 64B, which is what CopySlice will copy.

Checklist

• tested locally
• added automated tests
• updated the docs
• updated the changelog

[d-xo]

could be a little more general maybe, but otherwise lgtm

[msooseth]

I have now made it more general, let me know what you think! :)

[d-xo]

fine to merge as is, but wondering if we can have some recursive rewrites that work for buffers other than a single writeword over concretebuf?

[d-xo]

lgtm!