session builder: create pipeline from ELF files

[gballet]

Add a pipeline_from_elf method to the session builder, in order to be capable of creating sessions from a non-rust ELF riscv32imc file.

The proposed change is to still use guest_path as an input function, and change the behavior based on what item is pointed at by the path:

• if the path is a directory, it will try to compile it as a rust project
• if the path is a file, it will try to interpret it as a riscv ELF file

[gballet]

Tested and it worked (well, it failed on #2622, which means it loaded the file)