Convert the valid_* tuples to frozentuples.
Add the faust tool via Tool().

Add the faust tool via Tool() rather than directly as an argument to
Environment() in order to be able verify that FAUST is available using exists().
Only add -vec when using FAUST 1.
Add targets for generating SVGs.
Add the SCons faust tool as a submodule.

Add my modified version of the SCons faust tool as a submodule.
SCons: do out-of-source builds.

Change the build system to do out-of-source builds.  All the sources are in the
src/ subdirectory, and all of the output goes in the build/ directory.  The SVGs
are special-cased and go in a top-level svgs/ directory.
README: update compilation time notes.

Another change in the faust git repo optimised the definition of fir() et.al. to
compile faster, reducing the total compilation time further.
README: convert regular dash ("-") to "—".