Implement n() function (DuckDB)

[thisisnic]

library(dplyr, warn.conflicts = FALSE)
library(substrait, warn.conflicts = FALSE)

mtcars %>% 
  summarise(n()) %>%
  collect() 
#>   n()
#> 1  32

mtcars %>% 
  duckdb_substrait_compiler() %>%
  summarise(n()) %>%
  collect() 
#> Error: callr subprocess failed: rapi_prepare: Failed to prepare query CALL from_substrait('\x0a\x02\x08\x01\x12\x09\x1a\x07\x08\x01\x10\x02\x1a\x01\x6e\x1a\x95\x01\x12\x92\x01\x0a\x8a\x01\x22\x87\x01\x12\x7b\x0a\x79\x12\x66\x0a\x03\x6d\x70\x67\x0a\x03\x63\x79\x6c\x0a\x04\x64\x69\x73\x70\x0a\x02\x68\x70\x0a\x04\x64\x72\x61\x74\x0a\x02\x77\x74\x0a\x04\x71\x73\x65\x63\x0a\x02\x76\x73\x0a\x02\x61\x6d\x0a\x04\x67\x65\x61\x72\x0a\x04\x63\x61\x72\x62\x12\x2c\x0a\x02\x5a\x00\x0a\x02\x5a\x00\x0a\x02\x5a\x00\x0a\x02\x5a\x00\x0a\x02\x5a\x00\x0a\x02\x5a\x00\x0a\x02\x5a\x00\x0a\x02\x5a\x00\x0a\x02\x5a\x00\x0a\x02\x5a\x00\x0a\x02\x5a\x00\x3a\x0f\x0a\x0d\x6e\x61\x6d\x65\x64\x5f\x74\x61\x62\x6c\x65\x5f\x31\x1a\x00\x22\x06\x0a\x04\x08\x02\x2a\x00\x12\x03\x6e\x28\x29'::BLOB)
#> Error: Catalog Error: Scalar Function with name n does not exist!
#> Did you mean "|"?

^{Created on 2022-06-07 by the reprex package (v2.0.1)}