For consistency in the C API, avoid any initializations before
callling lua_lock.

Allow memory errors to be raised through the API (throwing the
error with the memory error message); error in external allocations
raises a memory error; memory errors in coroutines are re-raised
as memory errors.

Several of its callers needed that case and had to do the check
themselves.

In 'lundump.c', when loading the upvalues of a function, there can be
a read error if the chunk is truncated. In that case, the creation
of the error message can trigger an emergency collection while the
prototype is still anchored. So, the prototype must be GC consistent
before loading the upvales, which implies that it the 'name' fields
must be filled with NULL before the reading.

LUA_T* represents only types; tags (types + Variants) are represented
by LUA_V* constants.

Instead of an explicit value (field 'b'), true and false use different
tag variants. This avoids reading an extra field and results in more
direct code. (Most code that uses booleans needs to distinguish between
true and false anyway.)

In 'aux_rawset', top must be decremented after the barrier, which
refers to top-1. (Bug introduced in commit c646e57fd.)