The R lexer does not recognize:
    - any built-in functions,
    - any data structures,
    - all built-in constants,
    - exponents on floating points with decimals, but no mantissa

This update to `math.py` adds a list of only those functions contained
within `package:base`, output from R with via the `builtins(TRUE)`
function. Keywords for Keyword.Constant, Keyword.Namespace, and
Keyword.Type were manually subsetted from the resulting list. This is
the approach used by the Mathematica and Numpy lexers.

The update to `test.R` adds a sample of the builtin functions, data
structures, and namespace functions, along with an example of a floating
point with a decimal, but no fractional part.