[#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/grammar.go

@@ -0,0 +1,60 @@
+package policy
+
+import (
+	"github.com/alecthomas/participle"
+)
+
+var parser *participle.Parser
+
+func init() {
+	p, err := participle.Build(&query{})
+	if err != nil {
+		panic(err)
+	}
+	parser = p
+}
+
+type query struct {
+	Replicas  []*replicaStmt  `@@+`
+	CBF       uint32          `("CBF" @Int)?`
+	Selectors []*selectorStmt `@@*`
+	Filters   []*filterStmt   `@@*`
+}
+
+type replicaStmt struct {
+	Count    int    `"REP" @Int`
+	Selector string `("IN" @Ident)?`
+}
+
+type selectorStmt struct {
+	Count  uint32 `"SELECT" @Int`
+	Clause string `"IN" @("SAME" | "DISTINCT")?`
+	Bucket string `@Ident`
+	Filter string `"FROM" @(Ident | "*")`
+	Name   string `("AS" @Ident)?`
+}
+
+type filterStmt struct {
+	Value *orChain `"FILTER" @@`
+	Name  string   `"AS" @Ident`
+}
+
+type filterOrExpr struct {
+	Reference string      `"@"@Ident`
+	Expr      *simpleExpr `| @@`
+}
+
+type orChain struct {
+	Clauses []*andChain `@@ ("OR" @@)*`
+}
+
+type andChain struct {
+	Clauses []*filterOrExpr `@@ ("AND" @@)*`
+}
+
+type simpleExpr struct {
+	Key string `@Ident`
+	// We don't use literals here to improve error messages.
+	Op    string `@Ident`
+	Value string `@(Ident | String | Int)`
+}