Allow unnamed options in branches

[mjskay]

Unnamed options in branches should automatically be given names according to their values.

For example:

branch(
  some_option,
    0,
    3,
    "abc",
    x + 5
)

Should be equivalent to:

branch(
  trim,
    0 ~ 0,
    3 ~ 3,
    "abc" ~ "abc",
    "x + 5" ~ x + 5
)

That is:

• [x] numeric values are turned into numeric names
• [x] character values are turned into character names
• [x] expressions are turned into character names using something like rlang::expr_text()