Add shorthand macros for some conventient Lua string operations

[pmatilai]

Despite all the Lua magic we already do, it's annoyingly often the case that shelling out is easier (or at least shorter) than doing the same in Lua (substrings, length etc)

Add shorthand macros %gsub, %len, %lower, %rep, %reverse, %sub and %upper which simply wrap the corresponding Lua string.* functions for convenience.

[pmatilai]

Of course there's the question of whether it actually makes sense to do this in C at all, when wrapping them individually in macros file would be easier in many ways.

The other question is the naming: %sub here clashes with usage in our own test-suite, and if these clash here they may as well clash elsewhere. So I wonder if we should just add "str" as a common pre- or post-fix to all these...

[pmatilai]

Yet another question is whether it'd be saner to just do to do a native Lua call doString() directly. It'd be a few lines more probably but then we could give saner error messages, eg

$ ./rpm --eval "%sub x"
error: lua script failed: [string "sub"]:1: bad argument #2 to 'sub' (number expected, got no value)

The rest of the message is entirely reasonable but the "lua script failed" part is somewhat disorienting...

[pmatilai]

What, no bikeshedding on the names? 😆

[dmnks]

@pmatilai, the CI should now be fixed, please force push to this branch to trigger a new job.

[pmatilai]

The newly setup Semaphore project only catches new PR's so this is going to remain in limbo.
Since much to my shock nobody seems to be interested in bikeshedding over the names, I'll just merge the damn thing. "make ci" passes locally anyhow.

[hroncok]

The names are perfect BTW

[pmatilai]

I should take a screenshot of that comment and put it on the wall 😁