Thanks for adding note on non-requirement of constant time.

A minor point, which I should've brought up much earlier---the
argument order of string-replace! is different from srfi-13
string-replace; the latter takes src string as the second argument,
before the positions.
The order in this srfi may make more sense, but I'm afraid
that having different argument order in very similar named
procedures cause confusions in future.

>From: Per Bothner <xxxxxx@bothner.com>
Subject: Last call: Simple adjustable-size strings
Date: Sun, 26 Apr 2015 07:55:34 -0700

> I think we're ready to finalize:
> http://srfi.schemers.org/srfi-118/srfi-118.html
>
> I fixed some typos, and added some notes on execution speed.
> I've attached a diff relative to the previous version.
> --
> 	--Per Bothner
> xxxxxx@bothner.com   http://per.bothner.com/