minifer: mangle private names

For example,
```
export class Foo { #foo; #bar; method() { return this.#foo + this.#bar } }
```
should be minified to:
```
export class Foo{#e;#t;method(){return this.#e+this.#t}}
```

This requires another block-scoped symbol table (or another namespace in it that is minified separately), because inner private names can shadow outer private names.