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/parser.hh

@@ -0,0 +1,89 @@
+/*
+**  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.
+*/
+
+
+#ifndef __PARSER_HH__
+#define __PARSER_HH__
+
+#include <string>
+#include <vector>
+#include <memory>
+
+#include <parser/data.hh>
+#include <parser/tree.hh>
+#include <parser/proc-iface.hh>
+
+// A simple recursive-descent parser for queries. Follows the Xapian syntax,
+// but better handles non-alphanum; also implements regexp
+
+namespace Mux {
+
+/**
+ * A parser warning
+ *
+ */
+struct Warning {
+	size_t			pos; /**< pos in string */
+	const std::string	msg; /**< warning message */
+
+	/**
+	 * operator==
+	 *
+	 * @param rhs right-hand side
+	 *
+	 * @return true if rhs is equal to this; false otherwise
+	 */
+	bool operator==(const Warning& rhs) const {
+		return pos == rhs.pos && msg == rhs.msg;
+	}
+};
+
+
+/**
+ * operator<<
+ *
+ * @param os an output stream
+ * @param w a warning
+ *
+ * @return the updated output stream
+ */
+inline std::ostream&
+operator<< (std::ostream& os, const Warning& w)
+{
+	os << w.pos << ":" << w.msg;
+	return os;
+}
+
+/**
+ * Parse a query string
+ *
+ * @param query a query string
+ * @param warnings vec to receive warnings
+ * @param proc a Processor object
+ *
+ * @return a parse-tree
+ */
+using WarningVec=std::vector<Warning>;
+using ProcPtr = const std::unique_ptr<ProcIface>&;
+Tree parse (const std::string& query, WarningVec& warnings,
+	    ProcPtr proc = std::make_unique<DummyProc>());
+
+} // namespace Mux
+
+#endif /* __PARSER_HH__ */