all: Make file-scope variables internally linked where applicable (#57) ...

Narrows the scope of visible symbols to where they're actually used. Also
makes it easier to see true globals in source files
(ones used from multiple translation units)

se: Remove memset in se_perform_aes_block_operation (#53) ...

We can just initialize the array to be zeroed out. This is safer and less error-prone,
since the initializer is now associated with the variable directly, making it
impossible to put code relying on the zeroed out state before it (unlike with memset).

Move source files to src/, add Makefile, fix all build and linkage errors, etc.