Lazy-initialize the environment variables. (#184)

Lazy-initialize the environment variables. (#184)

* Lazy-initialize the environment variables.

This is the first in a series of PRs to make it easier to use WASI libc
in Wasm modules that don't have a `main` function. By initializing the
environment on demand, we avoid depending on having `__wasm_call_ctors`
run.

This uses weak symbols strategically to ensure that if `environ` is
used, it is initialized eagerly, but if only `getenv` and friends
are used, the environment is initialized lazily.

Eventually, I expect we'll have a convention for wasm modules without
main functions which will allow the `__wasm_call_ctors` function to be
called automatically, but this helps in simple cases for now.

Fixes #180.

* Add comments explaining the libc-environ-compat.h header usage.