Provide executables

[nielstron]

In order to improve accessibility even more, executables for opshin may be provided using pex.
In a fresh virtual environment with pex and opshin installed, the following produces a valid executable on linux:

$ pex opshin $(pip freeze) setuptools -c opshin -o ./dist/opshin
$ ./dist/opshin 
usage: opshin [-h] [-o OUTPUT_DIRECTORY] [--force-three-params] [--constant-folding]
              [--output-format-json] [--version]
              [{compile_pluto,compile,eval,parse,eval_uplc,build,lint}]
              [{spending,minting,rewarding,certifying,any,lib}] input_file [args ...]
opshin: error: the following arguments are required: input_file

Bug bounty: 3000 ADA