Data analysts should be able to use Text.replace to substitute parts of the text

[radeusgd]

Pull Request Description

Implements https://www.pivotaltracker.com/story/show/181266274

Important Notes

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[radeusgd]

These two additional mappings are used in replace but not in location_of.
For simplicity I always compute them - this is just creating 2 additional arrays along with the third one that is always created, so the cost shouldn't be awful. I prefer to keep it this way for simplicity instead of maintaining two pieces of code that do the same thing (which is prone to desynchronization).

Still, if anyone thinks that it's worth cutting the extra cost, I can make it into two separate functions. I just thought it is a premature optimization and decided to go for more readable code - we can always revisit it when we will have performance issues with Text and approach this correctly with comparative benchmarks instead of my gut instinct of what is important for perf. and what is not.

[jdunkerley]

Couple of minor things but other wise LGTM