This allows listeners to be notified when a CDATASection or
ProcessingInstruction is encountered during parsing. The non-listener
path still has the incorrect behavior of silently treating CDATASection
as Text nodes, but this allows listeners to handle them correctly.
Our floating point number parser was based on the fast_float library:
https://github.com/fastfloat/fast_float
However, our implementation only supports 8-bit characters. To support
UTF-16, we will need to be able to convert char16_t-based strings to
numbers as well. This works out-of-the-box with fast_float.
We can also use fast_float for integer parsing.
