Make local constant arrays static

[xenu]

Local, non-static constant arrays may get re-initialised at runtime (by pushing their contents on the stack) every time they are reached. This somewhat surprising behaviour is caused by the fact that the C standard requires (almost) every object to have a unique address.

In theory, a compiler may eliminate the re-initialisation, if it can prove that the address of an array is never examined. However:

1. This often isn't possible to prove. The compiler has no way to know what will happen to an array pointer once it leaves the compilation unit.

2. Unlike Clang, GCC and Visual C++ don't even attempt this optimisation at all. GCC has an open ticket for this:
   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59863

Making those arrays static avoids the problem, because static arrays are guaranteed to be initialised only once, prior the to the program startup.

• This set of changes does not require a perldelta entry.

[leonerd]

I see some callsites where sizeof has been changed to strlen - is that going to have a runtime cost on every evaluation now?

[xenu]

Compilers are smart enough to eliminate strlen for string constants. Here are outputs of GCC, Clang and Visual C++ (all with -O2): https://gcc.godbolt.org/z/1nr1snhrd

I think we already rely on this in a few places, ditto for memcpy.

You-know-who will probably complain that Visual C++ with -O1 can't. This is of course a self-inflicted problem, because we could easily switch to -O2 or at least -O1 -Oi. But you-know-who doesn't want to do that, and I don't have the energy to argue with him.

[xenu]

Force-pushed with a tiny change, I forgot one const in hangul_prefix declaration.

[khwilliamson]

I think we should have a section in perlhacktips of accumulated wisdom, maybe "Hints". This would be a good first entry.

[tonycoz]

Looks good otherwise.