Add =~ and !~ operators on strings

`left =~ right` return true if left contains right, using Rust's
`String::contains`. `!~` is the negated version.

A new `apply_operator` function is added which decouples evaluation from
`Value::compare`. This returns a `Value` and opens the door to
implementing `+` for example, though it wouldn't be useful immediately.

The `operator!` macro had to be changed slightly as it would choke on
`~` in arguments.

tests/filter_where_tests.rs

@@ -0,0 +1,112 @@
+mod helpers;
+
+use helpers as h;
+
+#[test]
+fn test_compare() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", h::pipeline(
+        r#"
+            open sample.db
+            | where table_name == ints
+            | get table_values
+            | first 4
+            | where z > 4200
+            | get z
+            | echo $it
+        "#
+    ));
+
+    assert_eq!(actual, "4253");
+
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", h::pipeline(
+        r#"
+            open sample.db
+            | where table_name == ints
+            | get table_values
+            | first 4
+            | where z >= 4253
+            | get z
+            | echo $it
+        "#
+    ));
+
+    assert_eq!(actual, "4253");
+
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", h::pipeline(
+        r#"
+            open sample.db
+            | where table_name == ints
+            | get table_values
+            | first 4
+            | where z < 10
+            | get z
+            | echo $it
+        "#
+    ));
+
+    assert_eq!(actual, "1");
+
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", h::pipeline(
+        r#"
+            open sample.db
+            | where table_name == ints
+            | get table_values
+            | first 4
+            | where z <= 1
+            | get z
+            | echo $it
+        "#
+    ));
+
+    assert_eq!(actual, "1");
+
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", h::pipeline(
+        r#"
+            open sample.db
+            | where table_name == ints
+            | get table_values
+            | where z != 1
+            | first 1
+            | get z
+            | echo $it
+        "#
+    ));
+
+    assert_eq!(actual, "42");
+}
+
+#[test]
+fn test_contains() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", h::pipeline(
+        r#"
+            open sample.db
+            | where table_name == strings
+            | get table_values
+            | where x =~ ell
+            | count
+            | echo $it
+        "#
+    ));
+
+    assert_eq!(actual, "4");
+
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", h::pipeline(
+        r#"
+            open sample.db
+            | where table_name == strings
+            | get table_values
+            | where x !~ ell
+            | count
+            | echo $it
+        "#
+    ));
+
+    assert_eq!(actual, "2");
+}