wp_apply often results in an of_val that should be simplified.

This patch was created using

  find -name *.v | xargs -L 1 awk -i inplace '{from = 0} /^From/{ from = 1; ever_from = 1} { if (from == 0 && seen == 0 && ever_from == 1) { print "Set Default Proof Using \"Type*\"."; seen = 1 } }1 '

and some minor manual editing

Also:

- Remove the wp_strip_later hack.
- Let wp_lam, wp_rec, wp_... initiate the proof mode.

I also renamed `iProof` into `iStartProof`, as it is supposed to be
something internal, and not a substitute of Coq's `Proof` command (as
originally intended).