Detect infinite dependencies

[emilyaherbert]

Closes #76

This PR adds code that checks for and detects infinite dependencies. In the event of circular or infinite dependencies, the compiler returns an error noting where the circular dependency is created.

[emilyaherbert]

Compiling infinite_dependencies
Compiled library "stdlib".
 --> /Users/emilyherbert/fuel/sway/test/src/e2e_vm_tests/test_programs/infinite_dependencies/src 
1 | library foo;
2 | dep b_dependency;                                                            
  | ^^^^^^^^^^^^^^^^^ File a_dependency.sw generates an infinite depedency cycle.
3 |

 --> /Users/emilyherbert/fuel/sway/test/src/e2e_vm_tests/test_programs/infinite_dependencies/src 
1 | library foo;
2 | dep a_dependency;                                                            
  | ^^^^^^^^^^^^^^^^^ File b_dependency.sw generates an infinite depedency cycle.
3 |

Aborting due to 2 errors.

[otrho]

I've thrown in a little commit which reversed the &mut BuildConfig changes, and now the diff is pretty much just the additions for the dependency graph and the internal check.