Rework logic to find module file:

Module is searched:
1) in current directory
2) in directory set by SQPATH environment variable
3) if that's not set, in $HOME/.squirrel/lib
4) if HOME is not set, in /usr/lib/squirrel

In each location, .nud, .nuc, .nut file is tried for binary,
bytecode, and source file respectively.
Rename low-level import function provide by this module to _import_().
Properly handle module not found condition (throw exception).
Add initial implementation of high-level module import support.

Implemented in Squirrel. Uses builtin dofile(), so works with source/bytecode
modules so far.
Add basic testsuite for high-level import functions.
Add defines to support building both dyna and static module from same source.
Add sample dummy module source.
Add AUTHORS file.
Add README.
Check that interpreter properly propagates script error to rc code.
Add more checks to import testsuite.
Preserve original proof-of-concept import implementation using dofile().
Convert high-level import to use low-level _import_() from binmodule.