Speed up the compilation process

[zinid]

• Do not parse source files twice while checking for relationship.
• Keep files relationships in a graph.
• The option 'keep_build_info' is introduced. When set to 'true' the graph will be kept in ebin/.rebar.build.info and will be used by further compiler calls. The default is 'false'.

[ghost]

Thanks @zinid, I'd like to ask some questions:

• Why didn't you enable keep_build_info by default? Is there a known issue?
• Do you have reproducible performance figures for comparison? Did you test a particular project?

[zinid]

@Tuncer

• No known issues. I'm thinking, probably, this feature should be set from a command line as this option is intended at developers mostly.
• ejabberd. After we replaced the build system with rebar the compilaton process became quite slow. I detect 2-fold speed-up with my patch: from 8 seconds to 4 seconds. Not much, this is just a consequence of removing redundant epp_dodger parsing. If 'keep_build_info' is enabled, it takes less than a second.

[zinid]

Ah, btw, the way I measured the compile time was:

timer:tc(rebar, main, [["skip_deps=true", "compile"]]).

[ghost]

What about adding a version marker and using the file for different purposes in the future?

[zinid]

Yes, storing version marker is a good idea.
As for different files for different purposes or one-for-all file: I don't know. It's always possible to write a converter if we have a version tag though.

[ghost]

You should match 'ok' here or else Dialyzer will complain.

[ghost]

@zinid, generally speaking I like the patch. Thanks a lot!

[ghost]

@zinid, @dizzyd, ping?

[Vagabond]

Closing, because it was subsumed by #129