Improve `Memory` API

[Robbepop]

• Adds Memory::{data_ptr, data_size}
• Changes return type of Memory::grow from Pages to u32
• Changes additional parameter type from Pages to u32
• Remove Memory::current_pages method
• Add Memory::size method

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 58.82353% with 7 lines in your changes missing coverage. Please review.

Project coverage is 80.48%. Comparing base (d088395) to head (dd4f849).

Files	Patch %	Lines
crates/wasmi/src/memory/mod.rs	56.25%	7 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1082 +/- ## ========================================== - Coverage 80.50% 80.48% -0.03% ========================================== Files 269 269 Lines 24992 24998 +6 ========================================== - Hits 20120 20119 -1 - Misses 4872 4879 +7 ```

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