[Proposal] Allow yields through string.gsub

["Soni L." <fakedme@...>]

With the recent changes to string.gmatch and the whole pattern matching 
code, it seems like now it's much easier to allow yields through 
string.gsub. May Lua users have yieldable string.gsub?

local function f(s, p) return string.gsub(s, p, coroutine.yield) end
local co = coroutine.wrap(f)
local s1 = co("test", ".")
print(v1) -- t
print(co("g"), co("o")) -- e s
local s2 = co("o")
print(s2) -- t
print(co("d")) -- good 4
co = coroutine.wrap(f)

Goals of this proposal:
1. Let coroutines be used for fast dynamic substitution, something we 
currently can't do.
2. string.gsub uses a buffer internally, while a string.gmatch-based 
approach would require either table allocation or string concatenation. 
For large source strings and small replacement strings, string.gsub 
would probably be better, both in memory, by cutting down allocation, 
and in CPU, by cutting down hashing and interning.
3. Event system interoperability (throw and handle events from a dynamic 
substitution).

Another option would be to just add buffers, which would, in theory, let 
you reimplement most of the standard libraries in pure Lua, with 
performance and memory characteristics closer to the C functions.

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