Smart strings

[sbarzowski]

When adding long strings, don't copy them immediately. Instead
build long strings only when their contents are requested.

This allows to build a long string from parts, using a regular
operator+ in linear time. This lets users to worry much less
about using std.join etc.

If indexing the string is mixed with building it using operator+
the behavior can still be quadratic. We may want to address it in
a later change.

Depends on: #229

[sbarzowski]

Example benchmark:

std.foldl(function(e, res) e + res, std.makeArray(20000, function(i) "aaaaa"), "")

This version: 0.73s user 0.13s system 131% cpu 0.652 total
Current master: 1.64s user 1.89s system 106% cpu 3.306 total
C++ version: 1.24s user 0.61s system 98% cpu 1.872 total

[coveralls]

Coverage increased (+0.2%) to 75.586% when pulling 093a766 on sbarzowski:smart-strings into 7af3b84 on google:master.

[coveralls]

Coverage increased (+0.05%) to 78.075% when pulling 8407897 on sbarzowski:smart-strings into b44e63e on google:master.

[sparkprime]

This is a good idea. We should tune it on some realistic benchmarks if possible.

I think having a native deep join that will do 2 passes over the tree will still be faster in the case of manifest functions, but obviously that is more of an expert approach.

[sbarzowski]

I rebased it and changed the names of string representations to something a little bit more concrete.

@sparkprime Can we merge this?