Document/define storage slots for reentrancy locks

[pcaversaccio]

We should define somewhere properly how the storage slots for reentrancy locks are chosen. In the compiler code, we can see this here. Also, I feel we should disallow empty reentrancy locks like @nonreentrant("") or @nonreentrant(" ") for safety reasons.

[fubuloubu]

It would be amazing to create an output storage slot table for viewing purposes, like in #1950

[charles-cooper]

nonreentrant keys are in -f layout output format.

$ vyper -f layout /dev/stdin <<EOF
@external
@nonreentrant("x")
def foo():
  pass
EOF
{"storage_layout": {"nonreentrant.x": {"type": "nonreentrant lock", "slot": 0}}, "code_layout": {}}

[charles-cooper]

invalid identifiers are prevented from being non-reentrancy keys in #3605