Restructure iic.py using subcommands

[alastairreid]

The iic.py frontend (previously called asl2c.py) has become a bit complicated to use since it supports

• generating a script of commands to control optimization/compilation/instrumentation
• generating C/C++/MLIR code
• compiling code to produce .o files
• linking code to produce .exe files
• running code
• generating metadata such as the callgraph and variables read/written by each function
• running tests

I think that this would be much simpler if it was restructured using subcommands (like git uses)

• isa-tool build
• isa-tool link
• isa-tool run
• isa-tool test
• ...

The names of subcommands should be aligned with similar tools such as Rust's 'cargo' https://doc.rust-lang.org/stable/cargo/commands/index.html