Speedup TypedData_Get_Struct

While profiling `Monitor#synchronize` and `Mutex#synchronize`
I noticed a fairly significant amount of time spent in
`rb_check_typeddata`.

By implementing a fast path that assumes the object is valid
and that can be inlined, it does make a significant difference:

Before:

```
  Mutex     13.548M (± 3.6%) i/s   (73.81 ns/i) -     68.566M in   5.067444
Monitor     10.497M (± 6.5%) i/s   (95.27 ns/i) -     52.529M in   5.032698s
```

After:

```
  Mutex     20.887M (± 0.3%) i/s   (47.88 ns/i) -    106.021M in   5.075989s
Monitor     16.245M (±13.3%) i/s   (61.56 ns/i) -     80.705M in   5.099680s
```

```ruby
require 'bundler/inline'

gemfile do
  gem "benchmark-ips"
end

mutex = Mutex.new
require "monitor"
monitor = Monitor.new

Benchmark.ips do |x|
  x.report("Mutex") { mutex.synchronize { } }
  x.report("Monitor") { monitor.synchronize { } }
end
```

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