PHP: Exchange: decimal_to_precision

[kornrunner]

Here's the PHP port of #1635, based on #2121

I've put new constants in the ccxt namespace to avoid polluting global state.
Constants have values based on #1635 (they differ in #2121).

I've tried to stick to the variables nomenclature (camelCase) based on #1635

There might be more uncovered / edge cases that need to be handled (negative precision isn't handled in existing PRs neither). Code is covered by same tests that already exist in previous PRs:

PHPUnit 6.5.7 by Sebastian Bergmann and contributors.

Runtime:       PHP 7.2.3
Configuration: /home/work/github/kornrunner/ccxt-origin/phpunit.xml.dist

.....................................................             53 / 53 (100%)

Time: 6.12 seconds, Memory: 4.00MB

OK (53 tests, 57 assertions)

I haven't checked it against previous versions (7.1, 7.0, 5.6 etc) - but this is all basic code without any new/shiny/fancy stuff. To be honest - code might be suboptimal - as I've mostly ported Py implementation to PHP. There might be better ways to achieve the same functionality.

Oh, and a bit offtopic: PHPUnit 6 is there because of PHP 5 support, as PHPUnit 7 requires PHP 7 and newer (and switching to newest versions is encouraged by whole community, but that's a topic for another thread).

Let me know if anything. Thank you.