Implement {glue} to construct paste/sprintf messages

[njtierney]

Slightly easier to maintain and use from a developer standpoint.

E.g.,

cat(
paste0(
  "\n\ngreta requires Python and several Python packages ",
  "to be installed, but no Python installation was detected.\n",
  "You can install Python directly from ",
  "https://www.python.org/downloads/ ",
  "or with the Anaconda distribution from ",
  "https://www.anaconda.com/download/"
)
)
#> 
#> 
#> greta requires Python and several Python packages to be installed, but no Python installation was detected.
#> You can install Python directly from https://www.python.org/downloads/ or with the Anaconda distribution from https://www.anaconda.com/download/

library(glue)

glue(
  "
  
  
  greta requires Python and several Python packages to be installed, but no \\
  Python installation was detected.
  You can install Python directly from https://www.python.org/downloads/ \\
  or with the Anaconda distribution from https://www.anaconda.com/download/"
)
#> 
#> 
#> greta requires Python and several Python packages to be installed, but no Python installation was detected.
#> You can install Python directly from https://www.python.org/downloads/ or with the Anaconda distribution from https://www.anaconda.com/download/

^{Created on 2021-04-21 by the reprex package (v2.0.0)}

and instead of

paste0(
        text,
        ifelse(is.null(text), "", " and "),
        "TensorFlow Probability isn't installed"

You write

glue("{text}, {ifelse(is.null(text), '', ' and '} TensorFlow Probability \\
     isn't installed")

[njtierney]

See also the glubort helper function that a lot of tidyverse pkgs seem to use:

https://github.com/DavisVaughan/slider/blob/579c3042d51fc48fb400d2dd6810a2677756e695/R/utils.R#L1-L3