OOM on concatenated strings - Fails to build projects with large files such as elm-css.

[rupertlssmith]

1. Check out rtfeldman/elm-css
2. guida make

Runs out of heap space due to concatenated strings.

Snippet contains the offending strings, formet by slicing the source code into snippets:

https://github.com/guida-lang/compiler/blob/master/src/Compiler/Parse/Primitives.elm#L228-L235

Instead of slicing the String up-front and putting it in the Snippet, just put in the whole unsliced String which will be a reference so only take up 1 pointer of memory. Then when code needs just the slice, apply the slice operation on demand, I have found String.slice in Elm to perform pretty well.

This is a Flyweight Pattern to share the String memory and use describe each snippet as a range into it.

Potentially the slice can be cached also, depends if all slices are materialized or only a small number are ever needed.

[decioferreira]

The following seems to be relevant to this: elm/bytes#19

[rupertlssmith]

Looks good with the read_string fix. elm-css compiling fine, and my 150 KLOC + 64 deps project not failing on OOM now.

Also profiled a run of make elm-css and the heap profile is greatly reduced. Didn't measure it but it did feel like compilation runs a little faster also.