fix src to be able to build the compiler (in linux)

[bcardiff]

I was able to repro the Hello Crystal sample in a ubuntu 16.04 + llvm 3.9 + wine 2.0-rc3 + the steps in https://github.com/crystal-lang/crystal/pull/3582 \o/ .

This PR should tidy up things a bit since someone need a crystal compiler with the windows target to emit llvm. Before this PR the compiler can't be built because the prelude is changed. This also make the build fail.

• restore content of prelude to be able to compile the compiler in linux with windows target.
• stub regex, iconv for windows
• add reference to io/memory due to path io/* expansion and migration to crystal 0.20.3
• add (temporal) windows-prelude.cr for easy shrink of samples in windows (require --prelude=empty).

# file ./foo.cr
require "./windows-prelude" 
puts "Hello Windows!" 

$ make # to build the compiler with windows target
$ ./bin/crystal build foo.cr --ll --single-module --cross-compile --target x86_64-windows-gnu --prelude=empty
$ wine /path/to/LLVM-3.9.0/bin/clang.exe  -g3 foo.ll -c -o foo.o
$ /path/to/mxe/usr/bin/x86_64-w64-mingw32.static-gcc -g3 foo.o -o foo.exe -liconv -lkernel32 -lgc -levent -lws2_32
$ wine foo.exe 
Hello Windows!

The windows-prelude.cr has the prelude.cr before this PR so your examples are reproducible, yet if someone uses further functionality it will most probably break.

If I am missing something in your workflow let me know.

[lbguilherme]

I didn't remember I had build the compiler with LLVM 3.8 locally before starting the changes and I was using this compiler instead of the prebuilt one for Linux (LLVM 3.5). This probably explains everything working for me with Clang 3.9.

Thanks for this. At least now the existing specs will pass.

[lbguilherme]

I've made just a change here: I moved windows-prelude.cr to src, so now one can write --prelude=windows-prelude instead of requiring in the source.