126 replace variable collapse with gluecollapse

[PietrH]

Resolves #126

Moves some objects that were only used for messaging inside glue syntax.

[PietrH]

Ready for review

[peterdesmet]

This PR moves the variables to the glue() function. It does not use glue_collapse() though. Do you see a benefit in using this over paste()?

[peterdesmet]

E.g. could we use:

assertthat::assert_that(
    all(!is.na(field_names)),
    msg = glue::glue(
      "All fields in `schema` must have property `name`.",
      "\u2139 Field(s) `{field_numbers}` don't have a name.",
      .sep = "\n",
      field_numbers = glue::glue_collapse(which(is.na(field_names)), "`, `")
    )
  )

[PietrH]

In this case I don't see a benefit, but not a real cost either. So we can certainly use glue_collapse() if you prefer.