Remove bcmath dependency

[terrafrost]

See https://github.com/phpseclib/bcmath_compat .

It basically turns phpseclib's BigInteger class into a BigDecimal class.

Surprisingly, the shim is actually faster than the original bcmath extension for certain operations. Using https://pastebin.com/x8wJBK4F (which, admittedly, is running off of phpseclib/phpseclib:dev-master - not off of the 2.0 branch) I get the following results:

• bcmath extension took 0.70488119125366 seconds
• phpseclib with PHP32 engine took 0.52849411964417 seconds
• phpseclib with BCMath engine took 0.48794078826904 seconds
• phpseclib with PHP64 engine took 0.23811507225037 seconds
• phpseclib with GMP engine took 0.0025088787078857 seconds

The fact that phpseclib is able to do modular exponentiation with bcmath faster than bcmath, itself, can is due to the fact that phpseclib (dev-master) uses barrett modular reduction whereas bcmath doesn't. And those results are without OpenSSL, which is used, as well, if it's available, for even faster modular exponentiation.

[lukebakken]

Please see #573. If that is a reasonable option, please fix the issues I found with that pull request, or open a new one to address your request here.

Remember that this library is maintained by volunteers, so contributing code (with tests) is the best way to help.

[terrafrost]

@lukebakken - see #695.

I'm not sure that #573 is really all that applicable. It looks like that is attempting to replace bcmath with floats. This just updates php-amqplib to use a bcmath polyfill.

Other famous polyfill's are ircmaxell/password-compat, paragonie/random_compat, paragonie/sodium_compat, ramsey/array_column, etc. Symfony has a whole collection of polyfill's, as well, for such things as mbstring, etc. They're supposed to be just drop in replacements for missing extensions or out-of-date PHP versions.

One thing that would be cool is Travis CI could run two versions of each PHP VM - one with bcmath and one without. But alas I do not know how to set it up to do that. I posted about this at https://stackoverflow.com/q/56318348/569976 in an attempt to figure out how this might be done.

[ramunasd]

@terrafrost Regarding travis, You can disable extension before tests and then run tests 2 times, with and without extension.

[lukebakken]

Moved to 3.x.x as we are waiting on an update to #695

[lukebakken]

Fixed by #754