Stream support (#812)

* Moves off of draining between filters. Instead, the sink will pull on the stream, and will drain element-wise. This moves the whole stream to being lazy.
* Adds ctrl-c support and connects it into some of the key points where we pull on the stream. If a ctrl-c is detect, we immediately halt pulling on the stream and return to the prompt.
* Moves away from having a SourceMap where anchor locations are stored. Now AnchorLocation is kept directly in the Tag.
* To make this possible, split tag and span. Span is largely used in the parser and is copyable. Tag is now no longer copyable.

tests/command_open_tests.rs

@@ -222,7 +222,7 @@ fn open_can_parse_utf16_ini() {
 fn errors_if_file_not_found() {
     let actual = nu_error!(
         cwd: "tests/fixtures/formats",
-        "open i_dont_exist.txt | echo $it"
+        "open i_dont_exist.txt"
     );
 
     assert!(actual.contains("File could not be opened"));