formalize grammar of relative time strings and use nom to parse them

BREAKING CHANGES: this commit introduced two breaking changes:

- Keyword `ago` semantic change (to be consistent with GNU date)

  The `ago` keyword now applies to individual date-time
  displacement (the one that immediately precedes it) and can
  be specified multiple times. For example, "1 day ago 2 days ago"
  gives a combined effect of "3 days ago", and "1 day 2 days ago"
  gives a combined effect of "1 day ago".

  In previous implementation, `ago` can only be specified once at
  the end of the relative time string, and it applies to all
  date-time displacements as a whole. So "1 day ago 2 days ago"
  is invalid and "1 day 2 days ago" gives a global effect of
  "3 days ago".

- Relative time strings like "lastweek" or "nextmonth" are now
  invalid, and space(s) are required in between the shift and the
  time unit.

Author: yuankunzhang

src/lib.rs

@@ -13,6 +13,8 @@ use regex::Regex;
 use std::error::Error;
 use std::fmt::{self, Display};
 
+pub(crate) mod parse;
+
 // Expose parse_datetime
 mod parse_relative_time;
 mod parse_timestamp;

src/parse/mod.rs

@@ -0,0 +1,19 @@
+// For the full copyright and license information, please view the LICENSE
+// file that was distributed with this source code.
+use relative_time::relative_times;
+
+mod relative_time;
+
+// TODO: more specific errors?
+#[derive(Debug)]
+pub(crate) struct ParseError;
+
+pub(crate) use relative_time::RelativeTime;
+pub(crate) use relative_time::TimeUnit;
+
+/// Parses a string of relative times into a vector of `RelativeTime` structs.
+pub(crate) fn parse_relative_times(input: &str) -> Result<Vec<RelativeTime>, ParseError> {
+    relative_times(input)
+        .map(|(_, times)| times)
+        .map_err(|_| ParseError)
+}