`mem_used()`

wraps around `gc()`

and returns the exact number of bytes
currently used by R. Note that changes will not match up exactly to
`obj_size()`

as session specific state (e.g. .Last.value) adds minor
variations.

`mem_used()`

prev_m <- 0; m <- mem_used(); m - prev_m#> 56,015,784 Bx <- 1:1e6 prev_m <- m; m <- mem_used(); m - prev_m#> 136,960 Bobj_size(x)#> 680 B#> 3,816 Bprev_m <- m; m <- mem_used(); m - prev_m#> 464 B