Improved performance of csv reader

by avoiding using with-input-from-port; it allocates a closure
and a handler chain, which happens at *every row*.
Make eof-object inlined
A pass5 optimization
Cosmetic changes
Support 'export' form without *.sci file

This is an internal matter---for precompiled extension modules, export
form will be included in *.sci file as a part of define-module form,
so the C part doesn't need to worry about exporting.  However, to build
the Gauche core, where we don't generate *.sci files, we do need exporting
in the C init function.
Splitted VM code generation API inoto gauche.vm.code module
Small stylistic changes