Breaking changes

The println builtin is now known as echo, shadowing the external echo.
Changes on comparison builtins:
- A family of string comparison builtins have been added, <s, <=s, ==s, !=s, >s and >=s (#238).
- The is builtin is the new generic (shallow) equality predicate. The deep equality predicate deepeq is now known simply as eq.
- The == builtin now compares numbers -- == 1 1.0 is true, while == a a throws an exception. To compare strings, use either ==s or is.
Automatic use'ing has been removed. All modules must be used before use.
The into-lines builtin is now known as to-lines.
The history builtin has been removed; use $le:history instead (#267).

Other notable changes

When run interactively, predicates that return false are now indicated with a ✗ sign.
Completion listing now has black text over a light background (#184), and is no longer unnecessarily tall (https://asciinema.org/a/87937).
Strings can now be indexed to obtain substrings (#243).
The peach command, a parallel version of each has been added (#244).
Multi-level modules are supported. For instance, the a:b module is the file ~/.elvish/a/b.elv (#249).
When exceptions are thrown, a full stack trace is now displayed (#207, https://asciinema.org/a/88801).
An unstable builtin -time has been added that shows the time to run a function.
An unstable builtin -override-wcwidth has been added to override wcwidth.
The navigation mode now supports previewing UTF-8-encoded text files (https://asciinema.org/a/89374).
A variable $le:after-readline has been added. It is a list of functions to be called just after the line editor completes reading input, with the input as the sole argument.
The auxiliary elvish-stub program has been removed, and elvish now always runs in foreground. As a nice side effect, elvish is go-gettable again.