Consider glue for wildcards

[wlandau]

https://github.com/tidyverse/glue. glue_data() should allow expansion over a grid of wildcards. Could either simplify/extend the internals or replace the wildcard interface entirely. Ref: #235.

[wlandau]

So glue is not likely to change how users supply wildcard placeholders. Curly braces are a good idea in principle, but they are not compatible with drake_plan()

drake_plan(x = my_{func}_tion())
#> Error: unexpected '{' in "drake_plan(x = my_{"

[wlandau]

Inside evaluate_plan(), we could just pre-wrap the wildcards in curly braces and then evaluate them with glue.

[wlandau]

After #235 (comment) and e5579b4, I am not sure glue is the answer. The main advantage at this point is performance, and wildcards are already a fast part of drake. I may revisit this.