Clarifications and hints regarding operators with type schemes, and clarification regarding the "any" type

Environment variable TEAL_CODEPROBER_MODE for forcing nicer defaults for CodeProber if someone gets the command line parameters wrong

Minor changes in Exercise0.md: changed all the occurrences of 'codeprober.sh' into 'code-prober.sh'. Changed 'localhost:8080' to 'localhost:8000'

Minor changes in README.md: changed all the occurrences of 'codeprober.sh' into 'code-prober.sh'. Changed 'localhost:8080' to 'localhost:8000'

The sources are still in the git history; the removal is purely
to reduce the risk of confusion.