Transpile to PHP 7.0 with Rector and manual replacements

[adamziel]

PHP 7.0 Compatibility

Makes the Blueprints library syntax–compatible with PHP >= 7.0 with automated transpilation and manual adjustments.

The automated transpilation is done with rector,
which downgrades features specific to PHP 7.2 and later features to PHP 7.1.

The manual part requires using regexps and editing the files by hand.

Automated part

To transpile the code to PHP 7.1, run:

# Install rector:
composer require rector/rector --ignore-platform-req=php

# Transpile:
php vendor/bin/rector process src

Unfortunately, Rector does not support downgrading to PHP 7.0 yet, so we need to do the
last stretch manually.

Manual part

Rector will downgrade PHP code to PHP 7.1 but not further. We need PHP 7.0 compat
so here's a few additional regexps to run. Regexps are not, of course, reliable in
the general case, but they seem to do the trick here.

List of manual replacements

• : \?[a-zA-Z_0-9]+ -> (empty string) to remove the unsupported return type
  from function(): ?SchemaResolver {} -> function() {}.
• : iterable to fix Fatal error: Generators may only declare a return type of Generator, Iterator or Traversable.
• \?[a-zA-Z_0-9]+ \$ -> $ to remove the unsupported nullable type from function signatures,
  e.g. function(?Schema $schema){} -> function($schema){}.
• (protected|public|private) const -> const as const visibility is not supported in PHP 7.0.
• : void -> `` as void return type is unsupported in PHP 7.0.
• [$ns, $name] = $this->parseName($name); -> list($ns, $name) = $this->parseName($name);
• foreach ($data as [$cp, $chars]) { -> foreach ($data as list($cp, $chars)) {
• Find or write Rector rules for downgrading to PHP 7.0

[adamziel]

There's plenty of formatting issues in this PR, let's reformat the entire repo in a follow-up one.