Miscellaneous stylistic improvements

[kilianmh]

• replace concatenate with concat
• replace nested if with single cond
• replace unless null with when
• replace if without else clause with when

[vindarel]

I was seeing an interest in calling the built-in concatenate, mainly to show the use of built-ins to new readers. Then it might be slightly more efficient, and more future-proof against a silly str:concat change. The other changes are good stylistic practice, this one less sure. What do you think?

[kilianmh]

show the use of built-ins to new readers

Does the function definition of str:concat not already show the use of concatenate to new readers of cl-str? ;)

slightly more efficient, and more future-proof against a silly str:concat change

In that case the users should quickly find out about concatenate in the hyperspec. Alternatively we could mention concatenate in a comment / docstring.

[kilianmh]

So then you want me to revert concat to concatenate for now so the other changes can be merged?

[vindarel]

So then you want me to revert concat to concatenate for now so the other changes can be merged?

yes please :)

[kilianmh]

Now it should be as discussed ;)

[vindarel]

Thank you again!