Notable new features

The os module has gained the following new commands: mkdir-all, symlink and rename.
A new render-styledown command.
A new str:repeat command.
A new md module, currently containing a single function md:show for rendering Markdown in the terminal.

Notable bugfixes

The string comparison commands <s, <=s, ==s, >s and >=s (but not !=s) now accept any number of arguments, as they are documented to do.

Support for the legacy ~/.elvish directory has been removed.
The commands !=, !=s and not-eq now only accepts two arguments (#1767).
The commands edit:kill-left-alnum-word and edit:kill-right-alnum-word have been renamed to edit:kill-alnum-word-left and edit:kill-alnum-word-right, to be consistent with the documentation and the names of other similar commands.

If you need to write code that supports both names, use has-key to detect which name is available:
```
fn kill-alnum-word-left {
    if (has-key edit: kill-alnum-word-left~) {
        edit:kill-alnum-word-left
    } else {
        edit:kill-left-alnum-word
    }
}
```
Using else without catch in the try special command is no longer supported. The command try { a } else { b } finally { c } is equivalent to just try { a; b } finally { c }.