@Kha When we write command macros, we can easily expand a command
into multiple ones by using `mkNullNode [cmd_1, ... cmd_n]`. Before
this commit, a tactic macro that expands into a sequence of tactics
had to produce `mkNullNode [tac_1, "; ", ..., "; ", tac_n]`.
I forgot the ";" a few times, and it produces very counterintuitive
behavior. This commit is the last step for fixing this issue.
The previous commits add the `withResultOf p f` combinator that allows
us to implement variants of the `unboxSingleton` trick. Now,
```
`(tactic| t)
```
Produces just `t` as before, but
```
`(tactic| t_1; t_2)
```
produces
```
Syntax.node `Tactic.seq [[t_1, "; ", t_2]]
```
instead of
```
Syntax.node `null [t_1, "; ", t_2]
```
cf85080e7a