Issue 4325: Replace most uses of scm_{from,to}_locale_* with fixed encodings

Issue 4325: Replace most uses of scm_{from,to}_locale_* with fixed encodings

Most are either identifiable as utf8, or as byte stream (more or less
called latin1) or pure ASCII.

Since conversion to C++ strings is factored through separate (and too
few) functions, those accesses are not yet reordered in a satisfactory
manner.