feat: vm.PrecompileEnvironment access to block info

[ARR4N]

Why this should be merged

Provides all extra block context required by stateful precompiles.

The warp precompile is dependent on a transformation (i.e. validation) of access lists, which is stored in types.Header.Extra, which is also made available.

Closes #23

How this works

vm.BlockContext has a *types.Header field added as an all-purpose carrier of context not otherwise included. Although there are many places that construct a BlockContext, core.NewEVMBlockContext() is in the hot path of most (and also has a types.Header argument) so it populates the new field.

A shallow copy of the header is available through vm.PrecompileEnvironment.BlockHeader(). Note that this returns an error if the BlockContext.Header pointer is nil. If this occurs then we can track the code path that led to creation of the BlockContext and populate it as necessary.

BlockNumber and BlockTime, although available via the Header are already in the BlockContext so are exposed via their own methods, neither of which return errors.

How this was tested

The existing integration test is extended to demonstrate proper plumbing of all newly accessible data. The makeOutput() function was becoming unwieldy so I abstracted it into a struct with a String() string method.