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]

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 ⚠️

Additional details and impacted files

@@            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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