Slightly better docs in pprinter
Show symbols as types, semantically

This makes them blue, like other types.
Fix precedence of application vs arrows in pprint

Changes
    Prelude.Applicative.<$> : Applicative f => (f (a -> b)) -> (f a) -> f b
into
    Prelude.Applicative.<$> : Applicative f => f (a -> b) -> f a -> f b
Improve docs for `so`

Now they include a new user warning.
Merge pull request #1185 from david-christiansen/pprint-stuff

Pretty-print fixes
Merge pull request #1186 from david-christiansen/docs

Improve docs for `so`
Show let bindings with proper line breaks and indentation in prover
Split :apropos queries at spaces, treat as conjunction

This makes it easier to narrow queries
Merge pull request #1188 from david-christiansen/pprint

Show let bindings with proper line breaks and indentation in prover
:apropos syntax for binders

Together with space splitting, this lets :apropos be used as a kind of
dumber Hoogle. For instance:

```
Idris> :apropos nat -> string
Prelude.Strings.length : String -> Nat
Returns the length of the string.

Idris> :apropos fin -> string

Idris> :apropos fin -> plus

Idris> :apropos fin -> +
Prelude.Fin.+ : Fin n -> Fin m -> Fin (n + m)
Add two Fins, extending the bound

Prelude.Fin.shift : (m : Nat) -> Fin n -> Fin (m + n)
Add some natural number to a Fin, extending the bound accordingly

Prelude.Fin.weakenN : (n : Nat) -> Fin m -> Fin (m + n)
Weaken the bound on a Fin by some amount
```
Document :apropos updates in CHANGELOG
Fix boolElim optimisation

It made assumptions about the form of Delay which have changed with the
new erasure, so wasn't being applied any more (it is a modest, but
noticeable, performance gain).
Merge pull request #1189 from david-christiansen/apropos-improvements

Apropos improvements
Add useful property to Prelude.Vect: xs ++ [] = xs