Add new file lang/spark (will become run-depends for GPS)

SPARK 2014 is a programming language and a set of verification tools
designed to meet the needs of high-assurance software development. SPARK
is based on Ada 2012, both subsetting the language to remove features that
defy verification, but also extending the system of contracts and aspects
to support modular, formal verification.

The new aspects support abstraction and refinement and facilitate deep
static analysis to be performed including information-flow analysis and
formal verification of an implementation against a specification.

SPARK is a much larger and more flexible language than its predecessor
SPARK 2005. The language can be configured to suit a number of application
domains and standards, from server-class high-assurance systems (such as
air-traffic management applications), to embedded, hard real-time,
critical systems (such as avionic systems complying with DO-178C Level A).

A major feature of SPARK is the support for a mixture of proof and other
verification methods such as testing, which facilitates the use of unit
proof in place of unit testing; an approach now formalized in DO-178C and
the DO-333 formal methods supplement. Certain units may be formally proven
and other units validated through testing.