Provides a module `assetcache` to support automatic cachebusting of
static web assets, rather than the current approach of incrementing
a version number.
On module load, it walks the file/assets/ dir, computes a CRC32 of
each asset, and provides those hashes via `assetcache_get(path)` at
runtime. The primary consumer of hashes at present is the new `asset`
filter for Jinja2, which accepts a relative asset path and returns the
full path, with cache busting, suitable for final use in the template.
Ex: `{{ 'css/main.css' | asset }}`
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