Rename 'user' to 'user_solver Z3Prover#5586

Issue Z3Prover#5586 reported that Android builds (targetting, e.g., x86) failed
to compile due to a conflict between:

* `struct user` in `sys/user.h`; and
* `namespace user` in z3's `user_solver.h`

This issue is resolved by renaming `namespace user` to `namespace
user_solver` (matching the header name) to avoid this conflict.

Reported-by: Jamie Collinson <jamiecollinson@gmail.com>

Signed-off-by: Andrew V. Jones <andrewvaughanj@gmail.com>

src/sat/smt/euf_solver.cpp

@@ -994,7 +994,7 @@ namespace euf {
         ::solver::push_eh_t& push_eh,
         ::solver::pop_eh_t& pop_eh,
         ::solver::fresh_eh_t& fresh_eh) {
-        m_user_propagator = alloc(user::solver, *this);
+        m_user_propagator = alloc(user_solver::solver, *this);
         m_user_propagator->add(ctx, push_eh, pop_eh, fresh_eh);
         for (unsigned i = m_scopes.size(); i-- > 0; )
             m_user_propagator->push();

src/sat/smt/euf_solver.h

@@ -101,7 +101,7 @@ namespace euf {
         sat::sat_internalizer* m_to_si;
         scoped_ptr<euf::ackerman>    m_ackerman;
         scoped_ptr<sat::dual_solver> m_dual_solver;
-        user::solver*          m_user_propagator = nullptr;
+        user_solver::solver*          m_user_propagator = nullptr;
         th_solver*             m_qsolver = nullptr;
         unsigned               m_generation = 0;
         mutable ptr_vector<expr> m_todo;

src/sat/smt/user_solver.cpp

@@ -18,7 +18,7 @@ Module Name:
 #include "sat/smt/user_solver.h"
 #include "sat/smt/euf_solver.h"
 
-namespace user {
+namespace user_solver {
 
     solver::solver(euf::solver& ctx) :
         th_euf_solver(ctx, symbol("user"), ctx.get_manager().mk_family_id("user"))

src/sat/smt/user_solver.h

@@ -23,7 +23,7 @@ Module Name:
 #include "solver/solver.h"
 
 
-namespace user {
+namespace user_solver {
 
     class solver : public euf::th_euf_solver, public ::solver::propagate_callback {