Optimize SplFixedArray dimension performance

[nielsdos]

This patch optimizes reading and writing from SplFixedArray with the dimension operators. It accomplishes this due to the following optimizations:

• Fast-path for long keys (inlined).
• Optimization hints (UNEXPECTED + assertion)
• Using an unsigned index so we can do a single length comparison

For the following script:

$test = new SplFixedArray(4);

for ($i = 0 ; $i< 5000000; $i++)
	$test[1] += $i;

On an i7-4790:

Benchmark 1: ./sapi/cli/php x.php
  Time (mean ± σ):      95.4 ms ±   1.6 ms    [User: 91.5 ms, System: 3.2 ms]
  Range (min … max):    93.7 ms … 100.8 ms    31 runs

Benchmark 2: ./sapi/cli/php_old x.php
  Time (mean ± σ):     119.1 ms ±   1.3 ms    [User: 114.7 ms, System: 3.6 ms]
  Range (min … max):   117.6 ms … 123.1 ms    24 runs

Summary
  ./sapi/cli/php x.php ran
    1.25 ± 0.03 times faster than ./sapi/cli/php_old x.php

On an i7-1185G7:

Benchmark 1: ./sapi/cli/php x.php
  Time (mean ± σ):      67.9 ms ±   1.1 ms    [User: 64.8 ms, System: 3.2 ms]
  Range (min … max):    66.6 ms …  72.8 ms    43 runs

Benchmark 2: ./sapi/cli/php_old x.php
  Time (mean ± σ):      84.8 ms ±   1.1 ms    [User: 81.0 ms, System: 3.9 ms]
  Range (min … max):    82.6 ms …  88.0 ms    34 runs

Summary
  ./sapi/cli/php x.php  ran
    1.25 ± 0.03 times faster than ./sapi/cli/php_old x.php

[Girgias]

Few questions, but makes sense overall.