Consistent conversion functions

[leighmcculloch]

The SDK contains many conversion functions from one type to another and so on. Some conversions are implemented as .to_*, .into_*, and .from_* functions, while others are implemented as Into/From impls, and then some are implemented as both.

I think we should strive to build some consistency with conversions so that every conversion has:

• A .to_ on the source type
• A .into_ on the source type if the source can be consumed and that's a meaningful optimisation
• A .from_ on the destination type
• A From impl on the destination type, or if not possible a Into impl on the source type, that call to .from_/.to_ above
• A From/Into impl on the borrowed type

Some of this naming and pattern is derived from the Rust Naming conventions for conversions:

Prefix	Cost	Ownership
as_	Free	borrowed -> borrowed
to_	Expensive	borrowed -> borrowed borrowed -> owned (non-Copy types) owned -> owned (Copy types)
into_	Variable	owned -> owned (non-Copy types)

Ref: https://rust-lang.github.io/api-guidelines/naming.html#ad-hoc-conversions-follow-as_-to_-into_-conventions-c-conv