gr: Improve conversions

C.f. https://flintlib.org/doc/gr.html#assignment-and-conversions

We can use more appropriate functions for conversions than roundtripping
through a string

Author: Tom-Hubrecht

src/flint/types/_gr.pxd

@@ -86,6 +86,8 @@ from flint.flintlib.functions.gr_domains cimport (
 )
 from flint.flintlib.functions.gr cimport (
     gr_heap_init,
+    gr_set_d,
+    gr_set_other,
     gr_set_str,
     gr_get_str,
     gr_set,
@@ -214,6 +216,24 @@ cdef class gr_ctx(flint_ctx):
         py_val._init = True
         return py_val
 
+    @cython.final
+    cdef inline gr from_d(self, double d):
+        cdef gr py_val
+        py_val = self.new_gr()
+        err = gr_set_d(py_val.pval, d, self.ctx_t)
+        if err != GR_SUCCESS:
+            raise self._error(err, "Incorrect conversion from a double")
+        return py_val
+
+    @cython.final
+    cdef inline gr from_other(self, gr x):
+        cdef gr py_val
+        py_val = self.new_gr()
+        err = gr_set_other(py_val.pval, x.pval, x.ctx.ctx_t, self.ctx_t)
+        if err != GR_SUCCESS:
+            raise self._error(err, "Incorrect conversion")
+        return py_val
+
     @cython.final
     cdef inline gr from_str(self, s: str):
         cdef gr py_val

src/flint/types/_gr.pyx

@@ -229,6 +229,13 @@ cdef class gr_ctx(flint_ctx):
         >>> ctx(2)
         2
         """
+        if isinstance(arg, gr):
+            return self.from_other(arg)
+        if arg is int:
+            return self.from_si(arg)
+        if arg is float:
+            return self.from_d(arg)
+        # TODO: fmpz & fmpq ?
         try:
             return self.from_str(str(arg))
         except AssertionError: