Fix ModuleGraph constructors

[plafer]

This should be done once #1226 is closed (specifically after all 4 points in #1226 (comment) are addressed)

Originally posted by @bobbinth in #1401 (comment)

For example, we could have the following constructors:

pub fn new(kernel: Kernel, libraries: IntoIterator<Item = CompiledLibrary>) -> Self {
    todo!("implement")
}

pub fn with_kernel(kernel: Kernel) -> Self {
    todo!("implement")
}

pub fn with_libraries(libraries: IntoIterator<Item = CompiledLibrary>) -> Self {
    todo!("implement")
}

And Assembler::default() could still be used to instantiate an empty assembler (i.e., the assembler without kernel or libraries).

[bobbinth]

Ah - I forgot about this issue and created #1433. I'll close this one as the new one has a more up-to-date description of the proposed constructors.