Minor fixes and QoL improvements (#360)

1. Bring `set-up-rust.sh` into sync with the README by making it executable (the README [expects it to be executable](https://github.com/verus-lang/verus#step-1-build-rust))
2. Correctly account for symlinks
    1. if the `rust/` directory is being pulled in as a symlink---useful if you're holding on to multiple copies of verus during development, where you want to share the main rust installation
    2. if the main verus repo itself is inside a symlinked folder

.gitignore

@@ -1,2 +1,2 @@
-/rust/
+/rust
 *.swp

tools/set-up-rust.sh

@@ -1,8 +1,9 @@
 #! /bin/bash
 
 toplevel=`git rev-parse --show-toplevel`
+curdir=`readlink -f "$PWD"`
 
-if [ "$toplevel" != "$PWD" ]; then
+if [ "$toplevel" != "$curdir" ]; then
     echo "ERROR: You should run this script at the root of the verus repository"
     exit 1
 fi

tools/update-rust.sh

@@ -1,6 +1,7 @@
 #! /bin/bash
 
 toplevel=`git rev-parse --show-toplevel`
+curdir=`readlink -f "$PWD"`
 python_command=''
 
 command -v python > /dev/null
@@ -10,7 +11,7 @@ if [ ! $has_python -eq 0 ]; then
     python_command='python3'
 fi
 
-if [ "$toplevel" != "$PWD" ]; then
+if [ "$toplevel" != "$curdir" ]; then
     echo "ERROR: You should run this script at the root of the verus repository"
     exit 1
 fi