Replace `digest()` with `rlang::hash()`

[gadenbuie]

rlang::hash() is quite a bit faster and we could remove one dependency by switching from digest::digest() to rlang::hash(). We use the hash to identify singletons:

htmltools/R/tags.R

Line 1033 in 487aa0b

sig <- digest(uiObj, "sha1")

htmltools/R/tags.R

Line 1063 in 487aa0b

sig <- digest(uiObj, "sha1")