[SPARK-21602][R] Add map_keys and map_values functions to R

[HyukjinKwon]

What changes were proposed in this pull request?

This PR adds map_values and map_keys to R API.

> df <- createDataFrame(cbind(model = rownames(mtcars), mtcars))
> tmp <- mutate(df, v = create_map(df$model, df$cyl))
> head(select(tmp, map_keys(tmp$v)))

        map_keys(v)
1         Mazda RX4
2     Mazda RX4 Wag
3        Datsun 710
4    Hornet 4 Drive
5 Hornet Sportabout
6           Valiant

> head(select(tmp, map_values(tmp$v)))

  map_values(v)
1             6
2             6
3             4
4             6
5             8
6             6

How was this patch tested?

Manual tests and unit tests in R/pkg/tests/fulltests/test_sparkSQL.R

[HyukjinKwon]

cc @felixcheung, could you take a look when you have some time?

[SparkQA]

Test build #80147 has finished for PR 18809 at commit d87f4c4.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[felixcheung]

LGTM.
@actuaryzhang would you like to review?

[actuaryzhang]

LGTM

[HyukjinKwon]

Thank you @felixcheung and @actuaryzhang.

[HyukjinKwon]

Merged to master.