Rewrite from proc-macro to build.rs

[marlonbaeten]

The current implementation uses a proc-macro. This macro is not expanded in incremental builds. This causes changes in the static asset directory to not show up in the binary.

This proposed change drops the load_assets! macro and introduces a build script for the memory-serve crate. The build script takes an environment variable to load assets from. Any changes in the path defined by ASSET_DIR or changes to the variable itself will trigger a rerun and will load fresh assets.

Note that this will be a breaking change:

MemoryServe::new(load_assets!("static"))

Becomes:

MemoryServe::new()

And the ASSET_DIR environment variable will be required.

This PR also ships some improvements to logging and improves build performance.