Indicate name of local variable for reads

Currently, nodes in concrete execution graphs can have labels like this:

1. x?0
2. y = $0
3. println($1)

This is a bit inconsistent. 1️⃣ uses the exact notation from the lecture, and reveals the actual value. 2️⃣ and 3️⃣ use a different notation (which is strictly necessary for 3️⃣) which reveal the names of (temporary) local variables, but do not show the actual value.

I suggest we switch everything over to a notation that shows the actual value:

1. x?0
2. y!0
3. println(0)

If this is not clear enough, the name of the local variable could always be added in gray:

1. x?0 [r]
2. y!0 [$0]
3. println(0) [$1]