Add name() function to lambda expressions that returns the measurement name of the point.

Adding this function will allow Kapacitor to manipulate the measurement name of the data like it can for fields and tags.

Something like this is what I have in mind:

```
// Convert the measurement to a tag
|eval(lambda: name())
.as('measurement')
.tags('measurement')
```

See https://community.influxdata.com/t/appears-there-is-a-kapacitor-memory-leak-with-child-process-udf/1668/2 for a discussion where this feature would have greatly helped

No files or tests are named. Start by locating lambda expressions and the existing field and tag accessors, then use the eval example as the behavioral guide; done means name() exposes the point's measurement name so it can be converted into a tag.

Written by the indexing model from the issue text.