[#23] Support SQL-like queries for placement

JSON format is rather verbose an inconvenient to be
edited by hand. This commit implements SQL-like
language for representing placement policy.

Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>

pkg/policy/doc.go

@@ -0,0 +1,20 @@
+// Package policy provides facilities for creating policy from SQL-like language.
+//   eBNF grammar is provided in `grammar.ebnf` for illustration.
+//
+// Current limitations:
+// 1. Grouping filter expressions in parenthesis is not supported right now.
+//    Requiring this will make query too verbose, making it optional makes
+//    our grammar not LL(1). This can be supported in future.
+// 2. Filters must be defined before they are used.
+// 	This requirement may be relaxed in future.
+//
+// Example query:
+// REP 1 in SPB
+// REP 2 in Americas
+// CBF 4
+// SELECT 1 Node IN City FROM SPBSSD AS SPB
+// SELECT 2 Node IN SAME City FROM Americas AS Americas
+// FILTER SSD EQ true AS IsSSD
+// FILTER @IsSSD AND Country eq "RU" AND City eq "St.Petersburg" AS SPBSSD
+// FILTER 'Continent' == 'North America' OR Continent == 'South America' AS Americas
+package policy