[xml] Initial implementation of core:encoding/xml.

A from-scratch XML implementation, loosely modeled on the [spec](https://www.w3.org/TR/2006/REC-xml11-20060816).

Features:
		- Supports enough of the XML 1.0/1.1 spec to handle the 99.9% of XML documents in common current usage.
		- Simple to understand and use. Small.

Caveats:
		- We do NOT support HTML in this package, as that may or may not be valid XML.
		  If it works, great. If it doesn't, that's not considered a bug.

		- We do NOT support UTF-16. If you have a UTF-16 XML file, please convert it to UTF-8 first. Also, our condolences.
		- <[!ELEMENT and <[!ATTLIST are not supported, and will be either ignored or return an error depending on the parser options.

TODO:
- Optional CDATA unboxing.
- Optional `&gt;`, `&#32;`, `&#x20;` and other escape substitution in tag bodies.
- Test suite

MAYBE:
- XML writer?
- Serialize/deserialize Odin types?

core/encoding/xml/debug_print.odin

@@ -0,0 +1,73 @@
+package xml
+/*
+	An XML 1.0 / 1.1 parser
+
+	Copyright 2021 Jeroen van Rijn <nom@duclavier.com>.
+	Made available under Odin's BSD-3 license.
+
+	A from-scratch XML implementation, loosely modeled on the [spec](https://www.w3.org/TR/2006/REC-xml11-20060816).
+
+	List of contributors:
+		Jeroen van Rijn: Initial implementation.
+*/
+import "core:fmt"
+
+/*
+	Just for debug purposes.
+*/
+print :: proc(doc: ^Document) {
+	assert(doc != nil)
+
+	using fmt
+	println("[XML Prolog]")
+
+	for attr in doc.prolog {
+		printf("\t%v: %v\n", attr.key, attr.val)
+	}
+
+	printf("[Encoding] %v\n",  doc.encoding)
+	printf("[DOCTYPE]  %v\n",  doc.doctype.ident)
+
+	if len(doc.doctype.rest) > 0 {
+		printf("\t%v\n", doc.doctype.rest)
+	}
+
+	if doc.root != nil {
+		println(" --- ")
+		print_element(0, doc.root)
+		println(" --- ")		
+	}
+}
+
+print_element :: proc(indent: int, element: ^Element) {
+	if element == nil { return }
+	using fmt
+
+	tab :: proc(indent: int) {
+		tabs := "\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t"
+
+		i := max(0, min(indent, len(tabs)))
+		printf("%v", tabs[:i])
+	}
+
+	tab(indent)
+
+	if element.kind == .Element {
+		printf("<%v>\n", element.ident)
+		if len(element.value) > 0 {
+			tab(indent + 1)
+			printf("[Value] %v\n", element.value)
+		}
+
+		for attr in element.attribs {
+			tab(indent + 1)
+			printf("[Attr] %v: %v\n", attr.key, attr.val)
+		}
+
+		for child in element.children {
+			print_element(indent + 1, child)
+		}
+	} else if element.kind == .Comment {
+		printf("[COMMENT] %v\n", element.value)
+	}
+}