Add an assert for max heap size

[mkacperek]

No description provided.

[rhempel]

The only problem I have with this is the call to assert() which can cause problems in some restrictive compile environments that do not support assert. I would propose two options:

1. Silently truncate the max heap size to the calculated limit
2. Silently create a minimal heap that is guaranteed to fail on the first allocation
3. Add user macros to allow behavior to be defined

The user macros could be:

UMM_HANG_IF_SIZE_TOO_BIG
UMM_TRUNCATE_IF_SIZE_TOO_BIG
UMM_MINMIZE_IF_SIZE_TOO_BIG

Honestly I would prefer the solution of making the note about maximum heap size a more visible warning in the docs - possibly verifying at compile time,

[rhempel]

I've made a change in the most recent release that handles this case (sort of) if you request a block bigger than the number of blocks, then we fail the allocation.
I think what I'll do here is take @evanrichters input and add a big comment to the init functing - and update the main docs with a more clear admonition