Add a 'clashi' executable

[thoughtpolice]

Currently, clash works by default in batch mode, and can be invoked in interactive mode with clash --interactive. This is a wonderful feature that puts it leagues ahead of most traditional HDLs, of course.

However, this is extremely tedious to type and I can feel my fingers crying every time I do it. A clashi executable that mimics ghci would be fantastic.

Pros:

• Incredible bloat decrease: only %20 more effort than clash, as opposed to 280% for clash --interactive. Amazing.

Cons:

• Kind of stupid to implement. I suppose A) Setup.hs for clash-ghc could install a script to do this for us; B) the clash-ghc package could have a new exe clashi that uses process to invoke clash --interactive <remaining args>, or C) somehow thread --interactive through the current Main.hs for clash-ghc, but this is complicated because doing that is GHC's responsibility when parsing arguments to create DynFlags.

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I think approach 2) (simply execve(2) the clash binary) is the right approach. (This is actually what GHC for Windows does, since it cannot use a shell script.)