Inline only trivial functions, such as `(empty? x)' -> `(null? x)',
to avoid generating too much code.
Bytecode includes a new `inline-variant' form, which records a
version of a function that is suitable for cross-module inlining.
Mostly, the variant let the run-time system to retain a copy
of the bytecode while JITting (and dropping the bytecode of)
the main variant, but it may be different from the main variant
in other ways that make it better for inlining (such a less loop
unrolling).
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