Add PHP static analyzers (of element types) to build checks?

Similar to #83

Motivation:

• More likely that phpdoc will remain accurate when refactoring
  • Improved autocompletion and error detection in projects using tolerant-php-parser
• Detect potential bugs in new commits/PRs that may be undetected by unit tests

There are several open source PHP static analyzers, such as https://github.com/phan/phan (which I maintain) and https://github.com/vimeo/psalm

• https://github.com/Microsoft/tolerant-php-parser/pull/176/files and https://github.com/Microsoft/tolerant-php-parser/pull/177/files were (the more obvious) issues detected using phan while I was working on a project to convert this to another AST format.

• An example of how this may be set up is https://github.com/TysonAndre/tolerant-php-parser/commit/daa503d776b52d609fa7b5fac010b6d6316df7c3 (example output at https://travis-ci.org/TysonAndre/tolerant-php-parser/jobs/277628702)
  (Both phan and psalm have ways to suppress categories of issues and individual issues)

No files or tests are named. Start by reviewing the repository's existing build checks, then compare the phan and psalm examples and linked analyzer-detected pull requests; define the analyzer, suppression policy, and CI success criteria before implementation.