Add new float host functions and versioning rules

[sappenin]

• Updated §5.8 (Floats) preamble to describe OpaqueFloat as a general-purpose 12-byte buffer (4-byte exponent + 8-byte mantissa), document the rounding_modes values, and drop the IOU-specific framing
• Added six new host functions to §5.8 (all at 1000 gas, consistent with existing float operations)
• Added §5.11 Host Function Versioning Rules, covering the immutability of deployed host functions and the requirement to introduce new function names if buffer layouts change

[pwang200]

Because a smart escrow should store the mantissa and exponent as separate integers in a contract-defined format if the smart escrow needs to persist the opaque float in the ledger. It needs a way to get the mantissa and exponent, through a new host function:

float_to_mantissa_and_exponent(
 opaque_float_in_buf: i32,
 opaque_float_in_len: i32,
 mantissa_out_buf: i32,
 mantissa_out_len: i32,
 exponent_out_buf: i32,
 exponent_out_len: i32,
)

Does it need a rounding mode? Not, right?

[sappenin]

@pwang200 I addressed your comment above via e56c1b9

[sappenin]

@pwang200 said:

Does it need a rounding mode? Not, right?

We decided that we don't, because the float is already rounded.