sibilant.compiler.SibilantCompiler gains a tmp_compiled context manager
sibilant.basics gains a defspecial to create a Special
sibilant.basics gains macro-let special which creates macros which are
only available during the compile-time of the body element. Syntax is
similar to the flet and labels macros.
Macros defined by macro-let are ONLY available at compile-time, not at
run-time. They do not declare local fast vars.
Closes #128