dillon1000/react

Rename SafeValue to ToStringValue (#13376)

Following up on the changes I made in #13367, @gaearon suggest that
"safe" could be read as necessary for security. To avoid misleading a
reader, I'm changing the name.

A few names where discussed in the previous PR. I think `ToStringValue`
makes sense since the value itself is not a string yet but an opaque
type that can be cast to a string. For the actual string concatenation,
I've used `toString` now to avoid confusion: `toStringValueToString`
is super weird and it's namespaced anyhow.

Definitely open for suggestions here. :) I'll wait until we wrap up
#13362 and take care of rebase afterwards.