Add Fallback for Systems without the MBString Extension

[dshanske]

https://github.com/indieweb/php-mf2/blob/4266673ca7883f55d9733204719feee95779df5f/Mf2/Parser.php#L96

We've had some issues with packages that depend on the parser running on sites where the hosting provider doesn't have the extension installed. Wondering if we could add a graceful failure at the parser level, rather than above it.

pfefferle/wordpress-semantic-linkbacks#122

[gRegorLove]

This would also require fallbacks for mb_substr and mb_strlen. What is WordPress' prior art for handling these fallbacks?

[miklb]

Perhaps SimplePie's example is relevant here

https://github.com/simplepie/simplepie/blob/master/library/SimplePie/Misc.php#L327

[dshanske]

mb_strlen and mb_substr actually have a compat function in WordPress.

https://core.trac.wordpress.org/browser/tags/4.8/src/wp-includes/compat.php#L45
https://core.trac.wordpress.org/browser/tags/4.8/src/wp-includes/compat.php#L100

[dshanske]

https://github.com/symfony/polyfill-mbstring

Seems this might work.