feat #1042 change md5 to murmurhash3, make it not depend on build time

1) Strip banner with AT release version and date before computing hash.

2) Change hash appended to built files' names from 128 bits (32 bytes) to 32 bits base36-encoded (<=7 bytes).

In practice, the risk of collision is still extremely low yet we gain a
few kilobytes less in output code size due to urlMap entries being shorter.

Close #1042.
Close #1060.