Use Buffer for huge XML support

[AlexC-ux]

I needed to generate an xml file with a total length greater than v8 allows (38 GB, and v8 only allows you to put 8 in a string), and these changes helped me not catch the “Range Error: Invalid string length” error.

All tests passed with the exception of 4 tests on error handlers, but this was unrelated to my changes.

If possible, I would like the library to support huge files.

[AlexC-ux]

@jjhbw @guigrpa please check this code when you find the time)

[jjhbw]

This looks like a solid optimization, thanks! 👍

I merged this offline in e1032d9.

I took out a few additional memory allocations in d94eb73.

The fact that we're using Buffer again also makes it necessary to restore the browser polyfill for Buffer (277cb58).