[CM] R7RS support

[bil at ccrma.Stanford.EDU]

> As a beginner, I ask: Do you approve?

Yes.

> (It also made me scared to use the R7RS features!)

Do you know if there is a test suite for the r7rs additions?

> To finish the job, I made the libraries load *implicitly* with …

Oh, I assumed the libraries were already loaded -- could you send
me the library-file code also? -- I'll update r7rs.scm -- thanks!