Decode UTF-32 (both byte orders, BOM or sniffed) and the single-byte encodings
ISO-8859-1 and Windows-1252, selected by the XML declaration's encoding name,
in addition to UTF-8 and UTF-16. The streaming byte path also handles UTF-32.
Standard library only, no Foundation. Closes#28. Reimplemented from the
documented encoding algorithms; no upstream code copied. 4 tests; 94 total.
Add PureXML.parse(pullingBytes:) and events(pullingBytes:) over an incremental
byte source (() -> UInt8?). StreamingDecoder detects the encoding from the
leading bytes, strips any BOM, and yields one scalar at a time as a Character,
so a byte stream is never fully buffered. UTF-8 (1 to 4 bytes) and UTF-16 (with
surrogate pairs) are decoded incrementally; invalid sequences become the Unicode
replacement character. Standard library only, no Foundation. Reimplemented from
the documented UTF decoding rules; no upstream code copied.
