experimental/hash

adding primop function calculating hash of a string

Signed-off-by: Marc Weber <marco-oweber@gmx.de>
Rename "hash" to "hashString" and handle SHA-1
Make "${./path} ..." evaluate to a string, not a path

Wacky string coercion semantics caused expressions like

  exec = "${./my-script} params...";

to evaluate to a path (‘/path/my-script params’), because
anti-quotations are desuged to string concatenation:

  exec = ./my-script + " params...";

By constrast, adding a space at the start would yield a string as
expected:

  exec = " ${./my-script} params...";

Now the first example also evaluates to a string.
Document ‘hashString’