non-boost build fixes (#756)

* Time.cpp: include <cmath>

Time.cpp uses `std::round` from <cmath>. When SLANG_USE_BOOST is on,
cmath is pulled in somehow from a transitive include. When
SLANG_USE_BOOST is off, at least on some systems, this include
is not pulled in.

Include <cmath> explicitly in Time.cpp to ensure `std::round` is
available, including for certain non-boost-using builds.

* pyslang.h: guard type_casters for flat_hash_map based on SLANG_USE_BOOST

include/slang/util/Hash.h defines the alias templates `flat_hash_map`
and `flat_hash_set` based on the macro SLANG_USE_BOOST. If the macro
is defined, these aliases refer to boost class templates. If the macro
is not defined, they reference standard class templates.

Pybind11 v2.10.4 defines type_casters for `std::unordered_map` and
`std::unordered_set` in include/pybind11/stl.h on lines
302-304 and 294-296, respectively.

As pyslang.h includes <pybind11/stl.h>, avoid multiple definitions
of these type casters by guarding their definitions in pyslang.h
based on the SLANG_USE_BOOST macro.

bindings/python/pyslang.h

@@ -167,6 +167,7 @@ struct type_caster<std::span<T>> {
     std::span<T> value;
 };
 
+#ifdef SLANG_USE_BOOST
 // Convert between flat_hash_map and python dict.
 template<typename Key, typename Value, typename Hash, typename Equal, typename Alloc>
 struct type_caster<flat_hash_map<Key, Value, Hash, Equal, Alloc>>
@@ -176,6 +177,7 @@ struct type_caster<flat_hash_map<Key, Value, Hash, Equal, Alloc>>
 template<typename Key, typename Hash, typename Equal, typename Alloc>
 struct type_caster<flat_hash_set<Key, Hash, Equal, Alloc>>
     : set_caster<flat_hash_set<Key, Hash, Equal, Alloc>, Key> {};
+#endif
 
 template<typename type>
 class type_caster<not_null<type>> {

source/numeric/Time.cpp

@@ -7,6 +7,7 @@
 //------------------------------------------------------------------------------
 #include "slang/numeric/Time.h"
 
+#include <cmath>
 #include <fmt/core.h>
 #include <ostream>