Add code snippets to 'Useful expressions' workspace

[hhzl]

From ML:

Add examples to 'useful expressions' workspace which show

T1. canonical examples of how to compress/decompress files/streams

T2. package loading through code.
Phil writes "I've been calling CodePackageFile
installFileStream:packageName:fullName: but that no longer appears to be
the correct entry point if I want dependency checking"

[hhzl]

Phil writes on the ML

"I started playing around with a couple of example test cases to see what
I ran into and came up with a distinct class to store all of these per
test category (i.e. under Test-Files a class ApiFile which could have a
method testWriteTextFile) The rationale was that it might make sense to
keep these test cases separate from traditional test cases which are
free to make calls that users of the class (i.e. who are calling the
supported API) should not."

[hhzl]

Check if other useful expressions in the workspace still apply.

[jvuletich]

The 'Useful Expressions' workspace included this since 2013:
'Example of loading a package with requirements on other packages'
Feature require: 'SignalProcessing'.

Just did a few tweaks to other entries there.

[hhzl]

Fine, let's then advertise the 'useful expressions' workspace as being updated and useful! (Reminder on the ML).