Rewrite gdb pretty-printer registration
Currently, the Rust pretty-printers are registered in gdb using the uninformative name "lookup":
(gdb) info pretty-printer
global pretty-printers:
[...]
objfile /home/tromey/[...]
lookup
It's nicer for users if the top-level registration is given a clear name. Additionally, gdb lets users individually enable and disable specific printers, provided they are registered correctly.
This patch implements both these ideas. Now the output looks like:
(gdb) info pretty-printer
global pretty-printers:
[...]
objfile /home/tromey/[...]
rust
StdArc
StdBTreeMap
StdBTreeSet
StdCell
StdHashMap
StdHashSet
StdNonZeroNumber
StdOsString
StdRc
StdRef
StdRefCell
StdRefMut
StdSlice
StdStr
StdString
StdVec
StdVecDeque