'port bump' does not update checksums for cargo crates

[lhaeger (Lothar Haeger)]

When updating a port that uses the cargo or cargo_fetch portgroups and has cargo.crates list required crates with their checksums, those will not be updated by port bump. I should be relatively easy to implement such an enhancement, though, and make maintaining cargo based ports a lot easier. Basically what has to be done today is this

1. clear cargo.crates
2. update the port version
3. run port bump
4. run port extract
5. run grep checksum ${worksrcdir}/Cargo.lock | perl -pe 's/"checksum (\S*) (\S*) \S* = "(\S*)"/ $1 $2 $3 \\/'
6. set cargo.crate to the output of 5. (without the trailing "\")

​https://github.com/macports/macports-ports/blob/master/_resources/port1.0/group/cargo_fetch-1.0.tcl has a few more details, but that's basically what needs to be added to port bumpif cargo.crates is set.