build: enable and fix -Wconversion,-Wsign-conversion

Fix instances of cppcoreguidelines-narrowing-conversions, and enable the
warnings to catch future cases.  Tested by building and running the unit
tests.

Change-Id: I4047acd87a5ac6050ed7650e1efd32a921574041
Signed-off-by: Brad Bishop <bradbish@qti.qualcomm.com>
diff --git a/meson.build b/meson.build
index 7945276..4508e20 100644
--- a/meson.build
+++ b/meson.build
@@ -23,7 +23,9 @@
     add_project_arguments(
         '-Wformat=2',
         '-Wcast-align',
+        '-Wconversion',
         '-Woverloaded-virtual',
+        '-Wsign-conversion',
         '-Wunused',
         '-Wduplicated-cond',
         '-Wduplicated-branches',