std: Move MemoryMap fields to methods

If modified, you can safely unmap arbitrary memory. These fields are not
intended to be modified, so read-only accessors are the only ones that are
provided.

Closes #15478
auto merge of #15588 : alexcrichton/rust/issue-15478, r=cmr

If modified, you can safely unmap arbitrary memory. These fields are not
intended to be modified, so read-only accessors are the only ones that are
provided.

Closes #15478