Base infrastructure for script generation, and operating system abstraction.

`defscript` defines an abstract script function, that can be implemented
for different operating system (and version) targets.

`implement` is used to add an implementation to a function defined with
`defscript`.

Dispatch of `defscript` is based on an implementation's specialisers vector.
All elements of the specialisers vector must match `*script-context*`.  The
specialisers vector may contain keywords that match if the are in
`*script-context*`, sets of keywords that match if any of the set values are
in `*script-context*`, or functions that will be passed `*script-context*` as
an argument and should return a truth value indicating whether a match
occured.

Mutiple implementations may match the `*script-context*` vector, and the
best fit is determined by the highest number of matching specialiser
functions, with ties decided by the earliest defined implementation.

Script library for abstracting target host script differences

Embed shell script in clojure.

Shell script is embedded by wrapping in the `script` macro.
(script (ls)) => "ls"

The result of a `script` form is a string.