Auto merge of #925 - christianpoveda:environ-shim, r=RalfJung
Auto merge of #925 - christianpoveda:environ-shim, r=RalfJung

Write name and value for each env var

In order to res0lve https://github.com/rust-lang/miri/issues/756 is necessary to have the whole `"NAME=VALUE"` sequence of bytes written into memory instead of just the value.

This change does not affect the interface of the `shim::envs::EnvVars` type in any way.

r? @RalfJung @oli-obk

rust-lang/miri,rust-lang/miri,rust-lang/miri,rust-lang/miri,tsion/miri