BREAKING: Drop IE11, Safari 9-10 support

BREAKING: Drop IE11, Safari 9-10 support

Since Buffer now supports the BigInt methods and the code for that uses the exponentiation operator, I'm going to drop support for IE11 and Safari 9-10.

Further changes I needed to make:

- Use consistent shifting to floor the offset numbers, like we do for other methods
- Remove all BigInt literals in favor of using a BigInt constructor – this fixes parse errors in older browsers like Safari 11
- Disable Node tests in Safari 11 and 12 since they use BigInt literal syntax

This commit contains fixes for PR #267