`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.

## Examples

```
prev_m <- 0; m <- mem_used(); m - prev_m
#> 61.38 MB
x <- 1:1e6
prev_m <- m; m <- mem_used(); m - prev_m
#> 87.15 kB
obj_size(x)
#> 680 B
rm(x)
prev_m <- m; m <- mem_used(); m - prev_m
#> 46.67 kB
prev_m <- m; m <- mem_used(); m - prev_m
#> 464 B
```