Allow heap feature to be disabled.

[ryankurte]

A while back the SDK introduced a heap allocator for everything but the nanos, which uses a config trick to enable this for all other platforms, however, the large size chosen for this default heap may exceed the available memory for applications that are already near RAM limits, and the target specific inclusion trick used to select this does not allow the heap feature to be disabled.

This PR simplifies the behaviour of the heap feature so this has no effect on the unsupported nanos target and can be disabled for apps that need to control the size of the allocated heap.

[yogh333]

LGTM but both ledger_device_sdk and ledger_secure_sdk_sys crates shall be version bumped (1.14.1 for ledger_device_sdk and 1.4.4 for ledger_secure_sdk_sys)

[agrojean-ledger]

@ryankurte @yogh333 @kingofpayne we need to be careful in allowing the heap to be disabled : the NBGL module relies on it for the alloc crate usage.

Apps for Stax and Flex that use NBGL (so most likely all of them) will not compile without the heap allocator enabled.

[yogh333]

@agrojean-ledger Yes you are right. This PR makes the heap feature enabled by default except on nanos so there is no issue on my side. To be disabled the --no-default-features shall be used explicitly. And when compiled for Nano S (no NBGL support), the heap feature is disabled automatically. So it is ok for me.

[agrojean-ledger]

@agrojean-ledger Yes you are right. This PR makes the heap feature enabled by default except on nanos so there is no issue on my side. To be disabled the --no-default-features shall be used explicitly. And when compiled for Nano S (no NBGL support), the heap feature is disabled automatically. So it is ok for me.

Ok then, up to developers to be careful when disabling the feature for a specific target.