Evaluate an expression and log results

`log_eval(expr, level = TRACE, multiline = FALSE)`

## Arguments

- expr
R expression to be evaluated while logging the expression itself along with the result

- level
`log_levels`

- multiline
setting to `FALSE`

will print both the expression (enforced to be on one line by removing line-breaks if any) and its result on a single line separated by `=>`

, while setting to `TRUE`

will log the expression and the result in separate sections reserving line-breaks and rendering the printed results

## Examples

```
if (FALSE) {
log_eval(pi * 2, level = INFO)
## lowering the log level threshold so that we don't have to set a higher level in log_eval
log_threshold(TRACE)
log_eval(x <- 4)
log_eval(sqrt(x))
## log_eval can be called in-line as well as returning the return value of the expression
x <- log_eval(mean(runif(1e3)))
x
## https://twitter.com/krlmlr/status/1067864829547999232
f <- sqrt
g <- mean
x <- 1:31
log_eval(f(g(x)), level = INFO)
log_eval(y <- f(g(x)), level = INFO)
## returning a function
log_eval(f <- sqrt)
log_eval(f)
## evaluating something returning a wall of "text"
log_eval(f <- log_eval)
log_eval(f <- log_eval, multiline = TRUE)
## doing something computationally intensive
log_eval(system.time(for(i in 1:100) mad(runif(1000))), multiline = TRUE)
}
```