Expose memory `needs_init` on `LinearMemory`/`RuntimeLinearMemory`

#### Feature

Need to have control over whether or not a linear memory is reinitialized when a module is instantiated.

#### Benefit

The use-case for this is to have persistent memory that does not get wiped out across runs.

#### Implementation

Expose a `needs_init` function on the `LinearMemory` trait. It can default to true, which will resort to the
current behavior of always reinitializing the memory when a module is instantiated.