Re-export the chrono crate from taskchampion.

The chrono types are central to use of TC, so this will help consumers
of the TC crate to avoid dependency conflicts.

Cargo.lock

@@ -1528,7 +1528,6 @@ dependencies = [
  "actix-web",
  "anyhow",
  "cc",
- "chrono",
  "env_logger 0.8.4",
  "lazy_static",
  "log",
@@ -3007,7 +3006,6 @@ dependencies = [
  "assert_cmd",
  "atty",
  "built",
- "chrono",
  "dialoguer",
  "dirs-next",
  "env_logger 0.8.4",
@@ -3036,7 +3034,6 @@ name = "taskchampion-lib"
 version = "0.1.0"
 dependencies = [
  "anyhow",
- "chrono",
  "libc",
  "pretty_assertions",
  "taskchampion",