Read-only queries cannot be issued on a read-only &World

## Problem

Given a `&World`, I cannot issue a query that only reads from the world.

There are two possible methods on `World` for this: `query`, `query_filtered`, plus `SystemState::new()`. All of them require `&mut World`, as they *could* provide mutating type parameters.

## Proposed Solutions

### Quick-and-dirty

Make read-only variants of all three methods.

### Ergonomic

Add a `system_state` and `read_only_system_state` method to `World`, which generates a `SystemState` and then immediately calls `.get_mut` / `.get` on it.

This allows users to do things like:

```rust
let query = world.read_only_system_state::>>();

for (foo, bar) in query.iter(){

}
```