Default compiler per project

[propensive]

Also fixes a problem with modules hashing to the same value if they only differ in their source directories.

The behavior for default projects is:

• if a project has a default compiler, new modules created in that project will default to that compiler
• if a project does not have a default compiler specified, the first new module added with a compiler specified will set the default project compiler too
• a project's default compiler can be set manually