Reduce warnings emitted by clang

This add the following changes to ongoing work on reducing the number of warnings emitted by clang:

[x] Explicitly named stdint integers, e.g. uint16_t over unsigned int in memory.h
[x] Memory 28-bit addresses changed from signed long to unsigned uint32_t in memory.h
[x] Explicitly pack structs on clang (for llvm-mos-sdk version v2.0.0 or earlier, see fix in https://github.com/llvm-mos/llvm-mos/commit/09aa314634ec1a3b9c56fdf8f19fc8bd71fa547d)
[x] Add test-integer test target
[x] Slightly stronger test of dma_poke() and dma_peek()
[x] Removed redundant includes on cc65
[x] Return named error instead of -1 in sdcard (see #37).
[x] Partial replacement of implicit casts with explicit ones

Closes #37.

MEGA65 / mega65-libc