Add additional build target which optimizes for size (#4096)

OpenWRT is a project which builds and distributes firmware for
embedded devies like routers, access points, and so on. These
devices are usually very limited in terms of storage. Therefore,
optimizing binaries for size at the cost of execution speed is
usually desired.

This PR adds an additional build-target, namely "release-micro",
which implements several parameters which optimize in favor of
binary size.

The following parameters were chosen:
- opt-level "z": Optimize for size with disabled loop vectorization
- strip "symbols": Strip debuginfo and symbols from binary
- lto "fat": Enable link-time optimizations across all crates
- codegen-units 1: Disable parallelization of code generation to
  allow for additional optimizations
- panic "abort": Abort on Panic() instead of unwinding

All these build parameters significantly reduce the binary size
from >40MB to <15MB - the actual amount depends on the target
architecture.

We would like to upstream this new build target to keep our build
environment simple. Other projects which deploy vaultwarden on
size-constrained environments may benefit from this change too.

Signed-off-by: Christian Lachner <gladiac@gmail.com>