[perf]: address regressions in slide_improvements

[dshemetov]

Perf has 3-4x slowdown due to validate_tibble and as_environment in as_slide_computation. To address later.

See here.

[brookslogan]

Note that this slowdown is for tidyeval [epi_slide] only. I believe Dmitry found a speedup for at least formulas [on epi_slide, and no perf regression for epix_slide?].