secp256k1: Make field value mul int take uint8.

This modifies the MulInt method on a field value to take a uint8 instead
of a uint in order to make it harder to misuse it since it has a
precondition which requires to the caller to ensure internal overflows
are avoided.   In practice, the code currently only ever calls it with a
max value of 8, but in order to prepare for exporting the type, it's
nicer to make it harder to misuse.

dcrec/secp256k1/field.go

@@ -590,7 +590,7 @@ func (f *fieldVal) Add2(val *fieldVal, val2 *fieldVal) *fieldVal {
 //
 // The field value is returned to support chaining.  This enables syntax like:
 // f.MulInt(2).Add(f2) so that f = 2 * f + f2.
-func (f *fieldVal) MulInt(val uint) *fieldVal {
+func (f *fieldVal) MulInt(val uint8) *fieldVal {
 	// Since each word of the field representation can hold up to
 	// 32 - fieldBase extra bits which will be normalized out, it's safe
 	// to multiply each word without using a larger type or carry

dcrec/secp256k1/field_test.go

@@ -822,7 +822,7 @@ func TestFieldMulInt(t *testing.T) {
 	tests := []struct {
 		name     string // test description
 		in1      string // hex encoded value
-		in2      uint   // unsigned integer to multiply with value above
+		in2      uint8  // unsigned integer to multiply with value above
 		expected string // expected hex encoded value
 	}{{
 		name:     "zero * zero",