lib: implement new query parser

mu's query parser is the piece of software that turns your queries
into something the Xapian database can understand. So, if you query
"maildir:/inbox and subject:bla" this must be translated into a
Xapian::Query object which will retrieve the sought after messages.

Since mu's beginning, almost a decade ago, this parser was based on
Xapian's default Xapian::QueryParser. It works okay, but wasn't really
designed for the mu use-case, and had a bit of trouble with anything
that's not A..Z (think: spaces, special characters, unicode etc.).

Over the years, mu added quite a bit of pre-processing trickery to
deal with that. Still, there were corner cases and bugs that were
practically unfixable.

The solution to all of this is to have a custom query processor that
replaces Xapian's, and write it from the ground up to deal with the
special characters etc. I wrote one, as part of my "future, post-1.0
mu" reseach project, and I have now backported it to the mu 0.9.19.

From a technical perspective, this is a major cleanup, and allows us
to get rid of much of the fragile preprocessing both for indexing and
querying. From and end-user perspective this (hopefully) means that
many of the little parsing issues are gone, and it opens the way for
some new features.

From an end-user perspective:
- better support for special characters.
- regexp search! yes, you can now search for regular expressions, e.g.
      subject:/h.ll?o/
  will find subjects with hallo, hello, halo,  philosophy, ...

  As you can imagine, this can be a _heavy_ operation on the database,
  and might take quite a bit longer than a normal query; but it can be
  quite useful.

lib/parser/tree.hh

@@ -0,0 +1,104 @@
+/*
+**  Copyright (C) 2017 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
+**
+**  This library is free software; you can redistribute it and/or
+**  modify it under the terms of the GNU Lesser General Public License
+**  as published by the Free Software Foundation; either version 2.1
+**  of the License, or (at your option) any later version.
+**
+**  This library is distributed in the hope that it will be useful,
+**  but WITHOUT ANY WARRANTY; without even the implied warranty of
+**  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+**  Lesser General Public License for more details.
+**
+**  You should have received a copy of the GNU Lesser General Public
+**  License along with this library; if not, write to the Free
+**  Software Foundation, 51 Franklin Street, Fifth Floor, Boston, MA
+**  02110-1301, USA.
+*/
+
+#include <vector>
+#include <string>
+#include <iostream>
+
+#include <parser/data.hh>
+
+namespace Mux {
+
+// A node in the parse tree
+struct Node {
+	enum class Type {
+		Empty, // only for empty trees
+		OpAnd,
+		OpOr,
+		OpXor,
+		OpAndNot,
+		OpNot,
+		Value,
+		Range,
+		Invalid
+	};
+
+	Node(Type _type, std::unique_ptr<Data>&& _data):
+		type{_type}, data{std::move(_data)} {}
+	Node(Type _type): type{_type} {}
+	Node(Node&& rhs) = default;
+
+	Type			type;
+	std::unique_ptr<Data>   data;
+
+	static constexpr const char* type_name (Type t) {
+		switch (t) {
+		case Type::Empty:    return ""; break;
+		case Type::OpAnd:    return "and"; break;
+		case Type::OpOr:     return "or"; break;
+		case Type::OpXor:    return "xor"; break;
+		case Type::OpAndNot: return "andnot"; break;
+		case Type::OpNot:    return "not"; break;
+		case Type::Value:    return "value"; break;
+		case Type::Range:    return "range"; break;
+		case Type::Invalid:  return "<invalid>"; break;
+		default:
+			throw std::runtime_error ("bug");
+		}
+	}
+
+	static constexpr bool is_binop(Type t) {
+		return t == Type::OpAnd || t == Type::OpAndNot ||
+			t == Type::OpOr || t == Type::OpXor;
+	}
+};
+
+inline std::ostream&
+operator<< (std::ostream& os, const Node& t)
+{
+ 	os << Node::type_name(t.type);
+	if (t.data)
+		os << t.data;
+
+	return os;
+}
+
+struct Tree {
+	Tree(Node&& _node): node(std::move(_node)) {}
+	Tree(Tree&& rhs) = default;
+
+	void add_child (Tree&& child) { children.emplace_back(std::move(child)); }
+	bool empty() const { return node.type == Node::Type::Empty; }
+
+	Node			node;
+	std::vector<Tree>	children;
+};
+
+inline std::ostream&
+operator<< (std::ostream& os, const Tree& tree)
+{
+	os << '(' << tree.node;
+	for (const auto& subtree : tree.children)
+		os << subtree;
+	os << ')';
+
+	return os;
+}
+
+} // namespace Mux