return early when possible in spec checking

[simonpcouch]

We've managed to cut down on calls to check_spec_mode_engine_val() in many places, though in most cases we're dropped into there we can still return early. It's hard to troubleshoot exactly what went wrong with misspecified specifications, but since the parsnip model environment is our ground truth for when a specification is well-defined, we should first look to it to see if we can skip the remaining checks. For quick fits like stats::lm() on a small dataset, this makes parsnip about twice as fast!😮

With main dev:

library(parsnip)

bench::mark(
  total = fit(linear_reg(), mpg ~ ., mtcars),
  iterations = 25
)
#> # A tibble: 1 × 6
#>   expression      min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr> <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 total        7.65ms   8.04ms      120.    7.19MB     67.6

With this PR:

library(parsnip)

bench::mark(
  total = fit(linear_reg(), mpg ~ ., mtcars),
  iterations = 25
)
#> # A tibble: 1 × 6
#>   expression      min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr> <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 total         3.7ms   3.87ms      255.    7.12MB     48.5

^{Created on 2023-03-17 with reprex v2.0.2}

There are additional tests for this infrastructure in extratests—they are fine. :)

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