Restructure crate

[pleshevskiy]

Initially, I concentrated everything in the `core' module. The idea has matured a bit, and I want to distribute it into separate modules.

• core - will contain EString
• std - will contain implementations for standard types (bool, numbers, Option, etc.)
• struct - will contain predefined types (SepVec, Pair, Triple, etc.)
• agg - will contain aggregate functions (Sum, Product, etc)
• low - will contain low-level structures for more accurate string parsing

[brxken128]

I think this would be a good way to go about it.

[pleshevskiy]

I cannot use struct, because it's a reserved keyword. Can you suggest another name for this scope (predefined types (SepVec, Pair, Triple, etc.))?

[pleshevskiy]

nevermind) I renamed to structs

[brxken128]

nevermind) I renamed to structs

I thought this would be an issue! I used structs too, it's close enough :)

[pleshevskiy]

I forgot about aggregates! I have a plan to move aggregates to the agg module.

[pleshevskiy]

It has been restructured in #21