templatizing CopyStringChars/CopyLinearStringChars and adding start param

SpiderShim uses custom implementations of CopyStringChars and 
CopyLinearStringChars [1] with two enhancements:

1. Per the V8 API, they accept an additional "start" parameter that 
identifies the index in the src string from which to start copying 
characters.

2. They're templatized, with char16_t* and char* specializations of 
CopyLinearStringChars, so SpiderShim's String::Write implementation can 
call CopyStringChars for both types of destination buffers [2].

Would it be feasible to upstream those enhancements?

-myk

[1] 
https://github.com/mozilla/spidernode/blob/e9b2841/deps/spidershim/src/v8string.h#L52-L108
[2] 
https://github.com/mozilla/spidernode/blob/e9b2841/deps/spidershim/src/v8string.cc#L363-L392