fix(instrumenter): support anonymous function names

[nicojs]

Do a best-effort attempt to minimize side effects from instrumentation by maintaining the fn.name property when placing mutants in code.

Instrument these:

const foo = function () { }
const bar = () => {}
const Baz = class { }
const foo = { bar: function () { } }

Like this:

const foo = true? function foo () { }: ...;
const bar = true? (() => { const bar = () =>{}; return bar } {})() : ...;
const Baz = true? class Baz { } : ...;
const foo = true? function bar() {}

Fixes #2362