feat: define ExperimentalInternetComputer.performanceCounter : Nat32 -> Nat64;

[crusso]

Returns the current value of IC performance counter counter:

• Counter 0 is the current execution instruction counter, counting instructions only since the beginning of the current IC message.
  This counter is reset to value 0 on shared function entry and every await.
  It is therefore only suitable for measuring the cost of synchronous code.

• Counter '1' is the call context instruction counter for the current call context.
  For replicated message executing, this excludes the cost of nested IC calls (even to the current canister).
  For non-replicated messages, such as composite queries, it includes the cost of nested calls.
  The current value of this counter is preserved across awaits (unlike counter 0).

• The function (currently) traps if counter >= 2.

Consult Performance Counter for details.

Example:

import IC "mo:base/ExperimentalInternetComputer";
  ///
let c1 = IC.performanceCounter(1);
work();
let diff : Nat64 = IC.performanceCounter(1) - c1;

[luc-blaeser]

Looks good. Nice documentation.
(Regarding the underlying IC specification: Very special semantics for the counter 1, with different counting for replicated and non-replicated messages...)