VM restructure and refactor

[sofiazcoaga]

This PR includes some restructuring of the VM. The major changes are:

• Create a utils.rs file for functions read_file, get_char and disable_input_buffering.
• Move the VMState to its own file and make functions mem_read, mem_write and write_ixs_to_mem part of its methods implementation.
• Reuse the get_char util function in TRAP handles.

[DiegoCivi]

Looks good! Just a little comment. Since getting the PC is very common, maybe you could implement the method vm.get_pc() so you avoid doing vm.registers[PC.usize()] all the time.