Add not equal comparator (!=)

The `qb.Cmp` lacks `not equal` methods related to the operator `!=`
even though the documentation
https://docs.scylladb.com/getting-started/dml/#select-statement mentions
it.

Closes: #114

Signed-off-by: Pierangelo Di Pilato <pierangelodipilato@gmail.com>

qb/cmp.go

@@ -21,6 +21,7 @@ const (
 	cnt
 	cntKey
 	like
+	ne
 )
 
 // Cmp if a filtering comparator that is used in WHERE and IF clauses.
@@ -53,6 +54,8 @@ func (c Cmp) writeCql(cql *bytes.Buffer) (names []string) {
 		cql.WriteString(" CONTAINS KEY ")
 	case like:
 		cql.WriteString(" LIKE ")
+	case ne:
+		cql.WriteString("!=")
 	}
 	return c.value.writeCql(cql)
 }
@@ -105,6 +108,54 @@ func EqFunc(column string, fn *Func) Cmp {
 	}
 }
 
+// Ne produces column!=?.
+func Ne(column string) Cmp {
+	return Cmp{
+		op:     ne,
+		column: column,
+		value:  param(column),
+	}
+}
+
+// NeTuple produces column!=(?,?,...) with count number of placeholders.
+func NeTuple(column string, count int) Cmp {
+	return Cmp{
+		op:     ne,
+		column: column,
+		value: tupleParam{
+			param: param(column),
+			count: count,
+		},
+	}
+}
+
+// NeNamed produces column!=? with a custom parameter name.
+func NeNamed(column, name string) Cmp {
+	return Cmp{
+		op:     ne,
+		column: column,
+		value:  param(name),
+	}
+}
+
+// NeLit produces column!=literal and does not add a parameter to the query.
+func NeLit(column, literal string) Cmp {
+	return Cmp{
+		op:     ne,
+		column: column,
+		value:  lit(literal),
+	}
+}
+
+// NeFunc produces column!=someFunc(?...).
+func NeFunc(column string, fn *Func) Cmp {
+	return Cmp{
+		op:     ne,
+		column: column,
+		value:  fn,
+	}
+}
+
 // Lt produces column<?.
 func Lt(column string) Cmp {
 	return Cmp{