Use StringVector instead of Vector{UInt8}

This avoids extra allocation and copying for the in-memory string
parsing.

REQUIRE

@@ -1,2 +1,2 @@
 julia 0.4
-Compat 0.17.0
+Compat 0.24.0

src/specialized.jl

@@ -1,12 +1,13 @@
 # Specialized functions for increased performance when JSON is in-memory
+using Compat: StringVector
 
 function parse_string(ps::MemoryParserState)
     # "Dry Run": find length of string so we can allocate the right amount of
     # memory from the start. Does not do full error checking.
     fastpath, len = predict_string(ps)
 
     # Now read the string itself
-    b = Vector{UInt8}(len)
+    b = StringVector(len)
 
     # Fast path occurs when the string has no escaped characters. This is quite
     # often the case in real-world data, especially when keys are short strings.