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.
I should add, if there's more code that relies on the side of effect of `GenerateCode()` here, then it's another perf bug that needs to be fixed.