Bugfix/include what you use in /prim/fun

[rok-cesnovar]

Summary

This fixes a bunch of include-what-you-use issues in /prim. There are a lot of functions and files like the ones mentioned in #1704 with missing includes of stan/math/prim/fun/exp.hpp and the rest.
These functions dont have much issues in practice, because we usually include master header files that include all files. But that doesnt mean that this should be cleaned up.

I made a simple Python script that flagged these files which I then fixed.

Example on prim/fun/beta.hpp (comments stripped):

#include <stan/math/prim/meta.hpp>
#include <stan/math/prim/fun/lgamma.hpp>
#include <cmath>

template <typename T1, typename T2>
inline return_type_t<T1, T2> beta(const T1 a, const T2 b) {
  using std::exp;
  return exp(lgamma(a) + lgamma(b) - lgamma(a + b));
} 

There is a missing #include <stan/math/prim/fun/exp.hpp> here which this PR adds. And that is it.
It also does some smaller changes:

• removes some unnecessary includes that were flagged by the same script
• removes a few unnecessary local variables that were only used once
• fix one error message in mdivide_right_tri
• did a bit of alphabetical ordering of includes but not systematically

Tests

/

Side Effects

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Copyright holder: Rok Češnovar

The copyright holder is typically you or your assignee, such as a university or company. By submitting this pull request, the copyright holder is agreeing to the license the submitted work under the following licenses:
  - Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
  - Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[stan-buildbot]

---

Name	Old Result	New Result	Ratio	Performance change( 1 - new / old )
gp_pois_regr/gp_pois_regr.stan	4.96	4.97	1.0	-0.15% slower
low_dim_corr_gauss/low_dim_corr_gauss.stan	0.02	0.02	1.01	0.63% faster
eight_schools/eight_schools.stan	0.09	0.09	1.01	1.35% faster
gp_regr/gp_regr.stan	0.22	0.22	1.0	0.22% faster
irt_2pl/irt_2pl.stan	6.06	6.05	1.0	0.08% faster
performance.compilation	88.12	87.01	1.01	1.26% faster
low_dim_gauss_mix_collapse/low_dim_gauss_mix_collapse.stan	7.49	7.47	1.0	0.3% faster
pkpd/one_comp_mm_elim_abs.stan	20.53	20.68	0.99	-0.74% slower
sir/sir.stan	93.6	91.09	1.03	2.68% faster
gp_regr/gen_gp_data.stan	0.05	0.05	1.0	0.44% faster
low_dim_gauss_mix/low_dim_gauss_mix.stan	3.01	3.01	1.0	-0.0% slower
pkpd/sim_one_comp_mm_elim_abs.stan	0.32	0.32	0.99	-1.37% slower
arK/arK.stan	1.76	1.74	1.01	0.94% faster
arma/arma.stan	0.8	0.8	1.0	-0.05% slower
garch/garch.stan	0.58	0.59	0.99	-1.48% slower
Mean result: 1.00285317853

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Jenkins Console Log
Blue Ocean
Commit hash: 9d94901

---

Machine information

ProductName: Mac OS X ProductVersion: 10.11.6 BuildVersion: 15G22010

CPU:
Intel(R) Xeon(R) CPU E5-1680 v2 @ 3.00GHz

G++:
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 7.0.2 (clang-700.1.81)
Target: x86_64-apple-darwin15.6.0
Thread model: posix

Clang:
Apple LLVM version 7.0.2 (clang-700.1.81)
Target: x86_64-apple-darwin15.6.0
Thread model: posix

[mcol]

Looks good! Thanks for the little cleanups on locals and header ordering too!