Trac #34284: hardcode that curves have dimension 1

Example:
{{{#!sage
sage: proof.all(False)
sage: R.<x> = GF((2^521-1,2))[]
sage: C = HyperellipticCurve(R.random_element(degree=999))
sage: %time _ = C.jacobian()
verbose 0 (3879: multi_polynomial_ideal.py, groebner_basis) Warning:
falling back to very slow toy implementation.
verbose 0 (1082: multi_polynomial_ideal.py, dimension) Warning: falling
back to very slow toy implementation.
CPU times: user 2.67 s, sys: 9.99 ms, total: 2.68 s
Wall time: 2.72 s
}}}

All the time is spent on computing the dimension of the curve, and since
the base field is not suitable for Singular this is done using Sage's
own implementation.

By definition, curves have dimension one, so this is totally redundant
and can be avoided by adding an implementation of `.dimension()` to
`Curve_generic` that simply returns `1`.

This was pointed out here:

  https://github.com/jack4818/Castryck-Decru-SageMath#additional-monkey-
patch-for-fixing-the-dimension

With the patch, there is no more "toy implementation" warning and it's
fast:

{{{
CPU times: user 119 µs, sys: 2 µs, total: 121 µs
Wall time: 124 µs
}}}

URL: https://trac.sagemath.org/34284
Reported by: lorenz
Ticket author(s): Lorenz Panny
Reviewer(s): Julien Grijalva

Author: Release Manager

src/sage/schemes/curves/affine_curve.py

@@ -908,11 +908,7 @@ def projection(self, indices, AS=None):
                To:   Affine Space of dimension 3 over Rational Field
                Defn: Defined on coordinates by sending (x, y, z, w) to
                      (x, y, z),
-             Closed subscheme of Affine Space of dimension 3 over Rational Field
-            defined by:
-               c - 1,
-               b - 3,
-               a - 2)
+             Affine Curve over Rational Field defined by c - 1, b - 3, a - 2)
 
         ::
 

src/sage/schemes/curves/curve.py

@@ -38,6 +38,8 @@
 from sage.schemes.generic.divisor_group import DivisorGroup
 from sage.schemes.generic.divisor import Divisor_curve
 
+from sage.rings.integer import Integer
+
 class Curve_generic(AlgebraicScheme_subscheme):
     r"""
     Generic curve class.
@@ -113,6 +115,24 @@ def _latex_(self):
             polys = ', '.join(f'${latex(p)}$' for p in self.defining_polynomials())
             return fr"\text{{{ambient_type} curve over ${ring}$ defined by {polys}}}"
 
+    def dimension(self):
+        r"""
+        Return the dimension of the curve.
+
+        Curves have dimension one by definition.
+
+        EXAMPLES::
+
+            sage: x = polygen(QQ)
+            sage: C = HyperellipticCurve(x^7 + x^4 + x)
+            sage: C.dimension()
+            1
+            sage: from sage.schemes.projective.projective_subscheme import AlgebraicScheme_subscheme_projective
+            sage: AlgebraicScheme_subscheme_projective.dimension(C)
+            1
+        """
+        return Integer(1)
+
     def defining_polynomial(self):
         """
         Return the defining polynomial of the curve.