Rework and extend endian-specific conversion API

[chfast]

Closes #105.

This reworks and extends mostly the intx::be API for converting to/from bytes in big-endian order.

The usage is shown in https://github.com/ethereum/evmone/pull/131/files.

After prototyping the usage of new API, I propose further changes:

• Rename uint<N> to load<uint<N>>. In the end it is more consistent so see be::load<uint256>(...) instead of be::uint<256> and would allow to support other types like be::load<int256>.
• Rename uint_unsafe() to unsafe_load() or unsafe::load(). The later would allow hiding the "unsafe" mark with using namespace intx::be::unsafe.
• Rename store_unsafe() to unsafe_store() or unsafe::store().

[codecov-io]

Codecov Report

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The diff coverage is 100%.

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[axic]

@chfast is this closing #105 ?

[chfast]

@chfast is this closing #105 ?

Yes.

[axic]

I think the API is fine (more readable than before), haven't reviewed the code though.