Load ELF object files

[morganthomas]

Modify Valida to detect if the program file is a raw binary file or an ELF executable file. If the input is an ELF executable file, then load the code and data sections into the program ROM chip and the static data chip, respectively. Include in this repo an example ELF executable file for testing purposes, and an automated test of loading and executing that file.

Note: The first 32-bit word of an ELF file is 0x7F454C46 (big endian), whereas the first 32-bit word of a Valida raw binary executable is a Valida opcode. Since 0x7F45 is not a Valida opcode, we can automatically detect whether a file is an ELF file or a raw binary file by looking at the first 32-bit word of the file.

[morganthomas]

Reopening as there are still some things to fix here; need to not load non-code sections into program ROM, and need to load .strtab sections into static data.