feat #1042 deterministic hash for builds; change md5 to murmurhash3

1) Let all the files except bootstrap not include build date and version,
making it possible to have (nearly) deterministic hashes of packages
(they'll only change once a year if no content changes).

2) Change hash appended to built files' names from 128 bit (32 bytes) MD5
to 32 bits base36-encoded (<=7 bytes) MurmurHash v3.

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.