Managed AWS Lambda runtimes wrap the function's runtime entry using a script
specified by the `AWS_LAMBDA_EXEC_WRAPPER` environment variable, however this is
not performed by provided runtimes (`provided`, `provided.al2`, `go1.x`).

This adds an `init` function that re-startes the current process after wrapping
it in the wrapper to emulate the behavior of other lambda runtimes, although
this will cause initialization of some other go packages to be run twice, which
will have an impact on cold start times.

See also: https://docs.aws.amazon.com/lambda/latest/dg/runtimes-modify.html#runtime-wrapper

Fixes aws#523