SBI-Rt version v0.0.2

In this version we changed API style to trait based type parameters, which would make it easier to
check parameter types at runtime to reduce errors. If user choose to use integer-impls feature,
it would fall back to older style functions using integer types.

Added

• Trait based type parameter for all extensions
• Feature integer-impls to allow fast prototyping with sbi-rt crate
• Feature legacy to gate SBI legacy extension
• Documents on various functions

Modified

• Update sbi-spec to version 0.0.4, re-export Version structure
• Function probe_extension now returns an ExtensionInfo value
• Function pmu_num_counters returns a usize value