Bump version to 0.20.0

This change bumps the crate to version 0.20.0. The following notable
changes have been made since 0.19.0:
- Migrated most usages of SystemTime date times to chrono::DateTime
- Added Activity::time method for retrieving the time stamp of an
  account activity
- Marked several more types as 'non_exhaustive'
- Made price related attributes of Position type optional after
  announcement of breaking API change at Alpaca
- Added fractionable attribute to Asset type
- Switched to using tarpaulin for code coverage collection
- Formatted code base using rustfmt and checked in configuration
  - Added enforcement of code formatting style checks in CI
- Added CI checks for auto generated code documentation
- Removed time-util dependency

CHANGELOG.md

@@ -1,5 +1,5 @@
-Unreleased
-----------
+0.20.0
+------
 - Migrated most usages of `SystemTime` date times to `chrono::DateTime`
 - Added `Activity::time` method for retrieving the time stamp of an
   account activity

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "apca"
-version = "0.19.0"
+version = "0.20.0"
 edition = "2018"
 authors = ["Daniel Mueller <[email protected]>"]
 license = "GPL-3.0-or-later"