Add new gas metering method: mutable global + local gas function

[agryaznov]

Solves #11

In addition to method used beforehand (i.e. injection of calls to an imported gas function into metering blocks), this new method now available too, which:

• Injects an exported mutable global into the module, which tracks the gas left.
• Injects the local gas function which subtracts the gas spent from the global, and drops unreachable if out of gas.

This new method should make the resulting module more efficient in terms of execution time, as invoking an imported gas host function every metering block adds an overhead. Synchronization between the global and the runtime should be implemented on the execution engine side.

[agryaznov]

new benches is WIP (coming soon)

[athei]

Let's do not perform more work in the gas function as necessary.

[agryaznov]

With new wasmi 0.20 engine, the overheads of calling an imported function and a local function are roughly the same. So the performance wins of using the new gas metering method are in question, and should be sanity-checked for each particular execution engine. Benchmarks are added showing how this could be measured. Versus using an emulated host function they show up to 46% performance decrease for modules instrumented with the new gas metering method. However, these numbers should not be treated as the ultimate truth, as in a real world setup it deeply depends on the host function implementation and where the execution engine resides. Client-side execution engine is expected to have slower host functions, so that's could be a proper use case for the new gas metering method with a mutable global.

This problem:

Just figured out that applying current implementation of stack_limiter injector after gas_metering::mutable_global injector bloats the resulting module massively. The reason is that with current impl of stack_limiter its wraps every call to the local gas function with its stack cost metering instructions. For many_blocks.wasm the overhead is 10x times of original module size.

will be addressed in a follow-up PR.

[agryaznov]

Benches

version tested: 6f43fbc

with rustc 1.65, wasmi 0.20.0

fixture	host func	mut global	perf
coremark	20.81 s	20.20 s	+3%
recursive_ok	367 µs	585 µs	-37%
fibonacci_recursive	9.15 µs	13.56 µs	-33%
factorial_recursive	1.50 µs	1.98 µs	-24%
count_until	5.03 ms	8.13 ms	-38%
memory_vec_add	6.21 ms	8.45 ms	-27%
tiny_keccak	925 µs	1080 µs	-14%
global_bump	3.79 ms	7.03 ms	-46%