Reduce memory writes and short-lived allocations.

[cbiffle]

I noticed that hot parts of the program (allocation and evaluation) were doing more work than is strictly necessary, so I've removed some of it.

• Removed the memset from tcl_append_string, since all but one byte of it is immediately overwritten by strncpy. Just zeroed the terminating byte instead.

• Loosened the return type of tcl_string to char* (not const), since many uses of it mutate the returned buffer.

• Replaced the initializing copy in tcl_append_string with memcpy, which is slightly faster than strncpy in cases like this where the length is known in advance. (This is about a 5% improvement in my tests.)

• Altered tcl_append to not free the right-hand argument, after noticing that almost every use of it created an otherwise-unused copy.

• In TCMD handling, rely on tcl_list_at to detect an empty command by returning NULL. This avoids a second tokenization pass over the string in tcl_list_length.

• Avoid making a short-lived copy of the value in tcl_var, just take ownership of the pointer instead.

On a small collection of microbenchmarks, this improved performance by about 25-30% on my machine.

[cbiffle]

(Noticed your suggestion to run clang-format on PRs, updated the commit accordingly -- it's done something odd to the TCMD case block, but now it's consistent.)