The string directory is for optimized string functions, currently
it is just a skeleton with dummy code.

To allow subprojects other than math, the build system and directory
layout is changed: all math related code, tools and tests are under
the math directory now, new subprojects should be similarly self-
contained.

The top level Makefile design is still kept, but the subproject build
directories provide their own Dir.mk with the build rules for the
subproject. The user interface of config.mk is kept for now, in the
future subproject specific flags and make variables may be added for
finer grained control.

math/single contained code for systems without double precision fpu
and rem_pio2 is not used currently and likely will be designed
differently when double precision trigonometric functions are added.

Scripts are known to work with sollya 6.0.

Add a single makefile such that it will rarely need modifications
with regular source code changes.

The usual configure step for build environment detection is done
manually by editing config.mk, it is expected to be simple make
variable changes (mostly CFLAGS), later a simple configure script
can be added to generate config.mk if necessary.

Update the README.