Fix for Razor #84 - Optimize GetSourceLocation

GetSourceLocation is frequently called to determine the location mappings
between the original document and the generated code.

The old implementation did a number of ToString and replace operations to
simplify the math on tracking the position - which put it front and center
in our performance measurements - about 25% of all execution time in a
sampling profile of our perf test.

The new code tracks position as code is written, and avoids allocations.
After these changes GetSourceLocation doesn't show up in the profile.
suggestion? It's literally an 'index' into the string. To me `index` and `i` convey the same amount of meaning.