Proposal for SentinelInputRange

[Walter Bright]

A SentinelInputRange is an InputRange with the following additions:

1. a compile time property 'sentinel' that is the terminating value of the range
2. empty is defined as: empty = (front == sentinel)
3. it is not necessary for empty to be called before front

A C style 0-terminated string is an example of a SentinelInputRange.

The additions to std.range would be:

1. isSentinelInputRange(T) which returns true if T is a SentinelInputRange
2. a unittest
3. documentation of this

An addition to std.string would be a function that takes a char* and returns a 
SentinelInputRange.

Motivation:
1. easy conversion of C strings to ranges
2. necessary for a fast implementation of a lexer

Any takers?