Rust formatter: fix pretty-printing of mutable slices
Rust formatter: fix pretty-printing of mutable slices

The supplied regular expression did not match mutable types correctly:
while we are using the Python API of LLDB to register summary functions,
the regular expression syntax that is supposed to be used is Posix ERE
(https://llvm.org/doxygen/classllvm_1_1Regex.html) and not the one
implemented by https://docs.python.org/3/library/re.html.

vadimcn/vscode-lldb,vadimcn/vscode-lldb,vadimcn/vscode-lldb,vadimcn/vscode-lldb,vadimcn/vscode-lldb,vadimcn/vscode-lldb,vadimcn/vscode-lldb