minicrypto sign

[doublex]

The signing code is messy.
The setup function is not ptls_minicrypto_init_secp256r1sha256_sign_certificate() but the privatekey parser ptls_set_ecdsa_private_key().
Wrapped in ptls_minicrypto_load_private_key().
ptls_minicrypto_secp256r1sha256_sign_certificate_t is exported in minicrypto.h.

The call stack should be: ptls_minicrypto_sign_cert() -> load + parse private-key -> setup according to key OID.

Changes:
Rename: uecc.c to sign.c
Move: ptls_set_ecdsa_private_key() to sign.c
Inline: ptls_minicrypto_init_secp256r1sha256_sign_certificate() into ptls_set_ecdsa_private_key()
Move: ptls_minicrypto_secp256r1sha256_sign_certificate_t from minicrypto.h to sign.c
Add new API: ptls_minicrypto_init_sign_certificate() and ptls_minicrypto_dispose_sign_certificate()

This pull request breaks the current signing API.