Enhance mmap value store with dynamic pre-allocation, robust indexing, naming changes

[hallogameboy]

Issue #, if available:
N/A

Description of changes:

Enhance mmap value store with the following changes:

• Functionalities
  • Enable dynamic pre-allocation for both of greater row and column numbers
  • Let _BytesBatchGetterValPreAlloc return a 2D list with individual sliced memoryview so that users don't need to have afterward slicing.
• Data types
  • Switch n_sub_row and n_sub_col to uint64 to avoid potential large numbers of entries
• Naming
  • Change submatrix to batch_get to be consistent with key mappers
  • Change num_f32 to float32 to be consistent across the code
  • Change str_mmap to bytes_mmap to be accurate in terms of user-given inputs

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.