Support for ARMv4

[omarandlorraine]

From what I see, there's armv7::InstDecoder and armv8::InstDecoder already.

I'm interested in developing for the ARM7TDMI which is an ARMv4, so it would be great to have an armv4::InstDecoder in this crate. But I don't know enough about later ARMs to know if armv7::InstDecoder will do the trick for me.

[iximeow]

i haven't exhaustively checked but i'm pretty confident that for each of the execution modes, thumb/aarch32/aarch64, extensions are strictly supersets of whatever they've extended. so armv7::InstDecoder should decode instructions for v4 targets correctly (excepting a bug or two in thumb: #3)