Fix overlapping memcpy call in String::trim

[earlephilhower]

memcpy() is undefined when source and destination overlap. String::trim
uses it when shifting the string left to remove left padding.

Replace with memmove() which is always safe, even when overlapped.

[earlephilhower]

Same kind of error as seen in ::remove, overlapping source and dest (this is from an add'l host test started locally but not pushed yet):

==19180== 1 errors in context 1 of 1:
==19180== Source and destination overlap in memcpy(0x5bb9f60, 0x5bb9f64, 7)
==19180==    at 0x4C32513: memcpy@@GLIBC_2.14 (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==19180==    by 0x4BC705: String::trim() (WString.cpp:756)
==19180==    by 0x41AC2D: ____C_A_T_C_H____T_E_S_T____20() (test_string.cpp:24)
==19180==    by 0x44B135: Catch::FreeFunctionTestCase::invoke() const (catch.hpp:5874)
==19180==    by 0x4206C2: Catch::TestCase::invoke() const (catch.hpp:6779)
==19180==    by 0x446677: Catch::RunContext::invokeActiveTestCase() (catch.hpp:5473)
==19180==    by 0x445EB5: Catch::RunContext::runCurrentTest(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&) (catch.hpp:5445)
==19180==    by 0x4432AD: Catch::RunContext::runTest(Catch::TestCase const&) (catch.hpp:5284)
==19180==    by 0x447521: Catch::Runner::runTests() (catch.hpp:5603)
==19180==    by 0x449B3A: Catch::Session::run() (catch.hpp:5734)
==19180==    by 0x44988D: Catch::Session::run(int, char* const*) (catch.hpp:5714)
==19180==    by 0x42B887: main (catch.hpp:9278)
==19180==
==19180== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)