Support for literals in INSERT and UPDATE and comparisons

The 'value' interface represents a CQL value for use in a comparison,
update, or intitialization operation. A consistent interface for this
allows us to easily support specifying default-named, custom-named,
literal, and evaluated-function values in all these contexts.

Parameters to Func should probably also be values to support full
composition, but that would be a breaking change because Func's
properties are exposed.

The value interface could itself be exposed if we wanted to allow
clients to pass their own values to SetValue, etc, but for now it is a
package-internal abstraction.

BLA

qb/value.go

@@ -0,0 +1,31 @@
+// Copyright (C) 2017 ScyllaDB
+// Use of this source code is governed by a ALv2-style
+// license that can be found in the LICENSE file.
+
+package qb
+
+import "bytes"
+
+// value is a CQL value expression for use in an initializer, assignment,
+// or comparison.
+type value interface {
+	// writeCql writes the bytes for this value to the buffer and returns the
+	// list of names of parameters which need substitution.
+	writeCql(cql *bytes.Buffer) (names []string)
+}
+
+// param is a named CQL '?' parameter.
+type param string
+
+func (p param) writeCql(cql *bytes.Buffer) (names []string) {
+	cql.WriteByte('?')
+	return []string{string(p)}
+}
+
+// lit is a literal CQL value.
+type lit string
+
+func (l lit) writeCql(cql *bytes.Buffer) (names []string) {
+	cql.WriteString(string(l))
+	return nil
+}