Fix potential ODR violation due to weak symbols being overridden

[bobsayshilol]

Reproducer:

// g++ repro.cpp edlib.cpp -o repro

#include "edlib.h"
#include <cstdio>

struct Block {
  Block();
};

// Shouldn't be called because no instance of `Block` is created in this file.
Block::Block() { printf("This shouldn't be called\n"); }

int main() {
  EdlibAlignConfig config{};
  config.mode = EDLIB_MODE_SHW;
  EdlibAlignResult result = edlibAlign("a", 1, "b", 1, config);
  edlibFreeAlignResult(result);
}

# Note that the same result is observed when using clang++ too.
$ g++ repro.cpp edlib.cpp -o repro
$ ./repro
This shouldn't be called

The issue is that Block's constructor is exported as a weak symbol from whichever translation unit has the contents of edlib.cpp since it defaults to external linkage. To fix this we wrap the types in an anonymous namespace to give them internal linkage and hide them from any client code.