DHM: new functions to query the length of the modulus

Add two functions mbedtls_dhm_get_len() and mbedtls_dhm_get_bitlen() to
query the length of the modulus in bytes or bits.

Remove the len field: the cost of calling mbedtls_dhm_get_len() each time
it's needed is negligible, and this improves the abstraction of the DHM
module.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/ChangeLog.d/dhm-fields.txt b/ChangeLog.d/dhm-fields.txt
new file mode 100644
index 0000000..620e3dc
--- /dev/null
+++ b/ChangeLog.d/dhm-fields.txt
@@ -0,0 +1,7 @@
+Features
+   * The new functions mbedtls_dhm_get_len() and mbedtls_dhm_get_bitlen()
+     query the size of the modulus in a Diffie-Hellman context.
+
+API changes
+   * Instead of accessing the len field of a DHM context, which is no longer
+     supported, use the new function mbedtls_dhm_get_len() .