Store the fiber_serial in the EC to allow inlining

Mutexes spend a significant amount of time in `rb_fiber_serial`
because it can't be inlined (except with LTO).
The fiber struct is opaque the so function can't be defined as inlineable.

Ideally the while fiber struct would not be opaque to the rest of
Ruby core, but it's tricky to do.

Instead we can store the fiber serial in the execution context
itself, and make its access cheaper:

```
$ hyperfine './miniruby-baseline --yjit /tmp/mut.rb' './miniruby-inline-serial --yjit /tmp/mut.rb'
Benchmark 1: ./miniruby-baseline --yjit /tmp/mut.rb
  Time (mean ± σ):      4.011 s ±  0.084 s    [User: 3.977 s, System: 0.011 s]
  Range (min … max):    3.950 s …  4.245 s    10 runs

Benchmark 2: ./miniruby-inline-serial --yjit /tmp/mut.rb
  Time (mean ± σ):      3.495 s ±  0.150 s    [User: 3.448 s, System: 0.009 s]
  Range (min … max):    3.340 s …  3.869 s    10 runs

Summary
  ./miniruby-inline-serial --yjit /tmp/mut.rb ran
    1.15 ± 0.05 times faster than ./miniruby-baseline --yjit /tmp/mut.rb
```

```ruby
i = 10_000_000
mut = Mutex.new
while i > 0
  i -= 1
  mut.synchronize { }
  mut.synchronize { }
  mut.synchronize { }
  mut.synchronize { }
  mut.synchronize { }
  mut.synchronize { }
  mut.synchronize { }
  mut.synchronize { }
  mut.synchronize { }
  mut.synchronize { }
end
```

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