CDRIVER-504 add `bson_array_builder_t` (#1344)

* document and test `bson_uint32_to_string` behavior when input buffer is too small.
* add `ASSERT_BSON_EQUAL_BSON` test macro
* add `bson_array_builder_t`
* add `bson_array_builder_append_*` functions
* use `bson_array_builder_t`
Replace existing use of `bson_append_array_begin`.
bcon-speed.c was deliberately not modified to use `bson_array_builder_append_array_begin` to avoid changing performance.

* move tutorial `Appending BSON` example code to separate file
* add doc page for `bson_array_builder_t` and update doc examples
* link to `bson_array_builder_t` doc page
* colocate `BSON_APPEND_*` macros with functions
Intends to make it easier to spot if a macro is missing.
* add `BSON_APPEND_ITER` and `BSON_APPEND_NOW_UTC`
* colocate `bson_append_*` with `bson_array_builder_append_*` functions
Intends to make it easier to spot if a function is missing.

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Co-authored-by: Adrian Dole <donald@dole.tech>