Add support for library build

[hanno-becker]

• Add a make lib build target, establishing libraries libmlkem.a (for all security levels) and libmlkem{512,768,1024}.a (for specific security levels). Each should include the FIPS-202 functionality. (Note: This means that libmlkem.a can not be obtained by linking together libmlkem{512,768,1024}.a -- which does not matter).
• Consider if it is possible to remove all symbols from libmlkemXXX.a apart from the top-level KEM API, which is what we want to expose and support (for now).
• Add test which builds an executable based on libmlkemXXX.a. This can either just replace the existing default test, or be a separate one.
• Extend README accordingly

[hanno-becker]

@potsrevennil I think this should be pretty easy after your work #457 -- can you do this? In my mind, this should take high priority. @mkannwischer What are your thoughts?