Make prompts and i-searches the same word length

`(reverse-i-search)` and `(i-search)` are different length prompts and so when switching search directions, the entire line shifts position.  I find it disorienting.  Fortunately, `reverse` and `forward` both have seven characters.  This very very minimal patch renames (i-search) to (forward-i-search).  It isn't how readline behaves, but I think it is better.

Similarly, `"help> "` becomes `" help> "` to keep the prompt start at the same location.
To fix issue #6874: updated the manual to reflect some subtleties
about type declarations and assertions with `::`.
This adds an expm1 function for complex values. It uses the property that

cos(x) - 1 = -2*sin(x/2)^2

There is some cancellation error when real(z) ~ imag(z)^2/2 for small z, but otherwise is fairly stable.
Merge pull request #6888 from mauro3/type_doc

To fix issue #6874: update of manual section on Types
add more tests
Merge pull request #6539 from simonbyrne/expm1

Adds an expm1 function for complex values.
formatting fixes
fix #6900, hashing dicts
more efficient hash for sets
Merge pull request #6893 from mbauman/stable-repl-prompt

Make prompts and i-searches the same word length