Improve tests & performance from latest release

[ichiriac]

Issue initialized here : #94

• Should improve coverage on latest 2.1.0 release
• Should improve performance by checking is('IDENTIFIER') at last
• Add a performance indicator test

[ichiriac]

Start a generic test, does not covers everything but can indicate if some generic behaviour becomes slower. Here my actual results, based on my virtual machine

  Performance tests
    + Tokens speed  => 1.13M/sec
    + Reading speed => 2.18M/sec
    ✓ tokenizer (118ms)
    + Nodes speed  => 137.58K/sec
    + Overall speed => 880.01K/sec
    ✓ parser (293ms)

[ichiriac]

Seems to be an overall boost ✨ 🎆 👍

New test takes 498sec to parse 48200 files :
https://travis-ci.org/glayzzle/php-parser-dev/builds/297672428#L889

From v2.1.0 it takes 620 sec :
https://travis-ci.org/glayzzle/php-parser-dev/builds/295681042#L896

And latest 2.0.7 takes 892 sec :
https://travis-ci.org/glayzzle/php-parser-dev/builds/271363662#L1290

May not run on same machine with same frequency, and may share CPU with other builds, so I can't conclude that's fully on code optimisation...