Fix length limit in ExternalStorage.get

[webmaster128]

Closes #126

1. Length limit for the result of ExternalStorage.get was increased to 128 KiB
2. Contracts that really really (really really111!!) need it, can use ExternalStorage.get_with_result_length. This is inconvenient by design as it is not included in the Storage trait.

The usability of this change in blockchains is limited by CosmWasm/wasmvm#59

[webmaster128]

The gas consumption changes slightly by this. This makes sense, since every 64 KiB the Wasm code needs to grow the dynamic memory by one page, which is a Wasm instruction.

---- instance::test::contract_deducts_gas_handle stdout ----
handle used: 59498
thread 'instance::test::contract_deducts_gas_handle' panicked at 'assertion failed: `(left == right)`
  left: `59498`,
 right: `59373`', packages/vm/src/instance.rs:381:9

I don't rebuild the test contract now to avoid conflicts with other open PRs. But it works locally.

[ethanfrey]

100 gas to change alloc from 2kb to 128kb seems quite cheap.
Happy with that cost so that no one can ever complain about too small data sizes.

[ethanfrey]

Happy if you can merge in my iterator PR and then rebuild the test contracts, then we should be good to merge.

[ethanfrey]

Looks good. And nice solution to the issue.

I would adjust some of the values, but otherwise good.

[ethanfrey]

Key_ptr can be FAR FAR less than 128KB.
I think making another constant for this and setting it to eg 2KB is more than overkill.

[webmaster128]

Is there any specific reason to make such assumptions? I mean, an application could choose to store all its data in keys with empty values. Why not? Just because this does not happen in the average case, should we make more restrictions than necessary?

In any case, I think it is good to have separate constants. Created

/// The number of bytes of the memory region we pre-allocate for the result data in ExternalIterator.next
static DB_READ_KEY_BUFFER_LENGTH: usize = 64 * KI;

now to prove the point that they are independent. Happy to change the value to something small if there is a good reason to.

[ethanfrey]

nice work