Review and simplify/improve `alloc_str` calls in the C backend

[osa1]

Currently if I have a string "Hi" in a program this is the C we generate:

alloc_str("Hi", 2)

alloc_str allocates a new memory and copies the string:

fir/src/to_c.rs

Lines 156 to 162 in d0047fc

	// Allocate string from bytes
	static Str alloc_str(const char* bytes, size_t len) {{
	uint8_t* data = malloc(len);
	memcpy(data, bytes, len);
	Array_U8 arr = {{ .data_ptr = data, .len = len }};
	return (Str){{ ._0 = arr }};
	}}

Instead of allocating a new memory we could reuse the static memory of the string literal as the data pointer here.

We should review all alloc_str calls to check whether the argument can be used as the data pointer, and whether we're properly freeing the argument after calling alloc_str.